Lean Production Systems 4.0: systematic literature review and field study on the digital transformation of lean methods and tools

Lean Production Systems are enterprise-specific, methodical frameworks for the continuous orientation of all enterprise processes to the customer in order to achieve overall objectives. Due to an increasing complexity of the digital transformation, the design of Lean Production Systems 4.0 is a challenging task for industrial engineering practice. For this, academia and industrial practice were analysed in a combined approach of a systematic literature review and a field study. The systematic literature review of 62 out of 1600 scientific papers shows that especially the tools, processes, and methods in Lean Production Systems are subject to digital transformation. By involving industrial practice in a field study, quantitative and qualitative insights were used to check scientific hypotheses and identify practical requirements by industrial engineers. As a result, 10 guidelines for the design of future Lean Production Systems 4.0 are derived. The practical implications of this paper enhance the evolution of enterprise-specific Lean Production System 4.0 frameworks.


Introduction
A production system is a socio-technical system that transforms inputs into outputs through value-adding processes as part of the transformation process (Nyhuis et al. 2008).Inputs include materials and used energy, as well as the necessary financial resources and the knowhow employed.Outputs include the resulting products and other residual materials.The task of a production system is to provide a final or intermediate product for customers (Nyhuis et al. 2008).
Production systems have developed via Taylorism, the Toyota Production System and Lean Production in the twentieth century.Through Taylorism, industrial work was transferred to economical mass production with methodical standardisation and rationalisation measures.Ford developed the automobile industry's first comprehensive production system for mass production (Taylor 1911;Spath 2003;Dombrowski and Mielke 2015).The Toyota Production System (TPS) is a holistic overall system of methods to shorten lead times, reduce costs, and increase quality.The TPS is founded on the elimination of waste and the continuous improvement process.The achievement of goals in TPS is made possible by the two pillars of Just in Time and Autonomous Automation (Jidoka) ( Ōno 1988;Dombrowski and Mielke 2015).
Lean Production, or Lean Manufacturing, has introduced methodical components of TPS to management practices of production systems and other business areas in the Western world (Womack and Jones 2003;Womack, Jones, and Roos 1990;Liker 2021;Shah and Ward 2007;Dombrowski and Mielke 2015).Building upon the concept of 'factory physics' by Hopp and Spearman (1996), Treville and Antonakis (2006) propose the following definition for Lean Production: 'Lean production is an integrated manufacturing system that is intended to maximise the capacity utilisation and minimise the buffer inventories of a given operation through minimising system variability (related to arrival rates, processing times, and process conformance to specifications).' (Treville and Antonakis 2006) The German guideline for Lean Production Systems (LPS) by VDI (2012) consolidates the developments of TPS and Lean Production and, at the same time, expands the definition of production systems by integrating the concept of holism in a company-wide understanding.VDI (2012) was published by a relevant standardisation institution and is the comprehensive development of a working group with leading researchers from production engineering, building upon the approved international LPS literature, e.g.Ōno (1988), Shingō (1989), Kobayashi (1995), Cochran et al. (2001), Liker (2004), Spear and Bowen (2008), Bozdogan et al. (2000).According to VDI (2012), a Lean Production System is defined as 'an enterprise-specific, methodical system of rules for the continuous orientation of all enterprise processes to the customer in order to achieve the targets set by the enterprise management.'The general structure of Lean Production Systems is composed of the five levels objectives, processes, principles, methods and tools, while the content within these levels is company-specific (VDI 2012).An overall optimum is not necessarily reached by the application of individual methods and tools in enterprise processes (cf.Liker 2021; Roth and Steege 2014;Zollondz 2013;Treville and Antonakis 2006).'Sustained success is achieved only through the integration in an LPS, which specifies the selection and synchronisation of principles, methods, and tools and is understood, accepted, and implemented by all employees on all levels of the enterprise' (VDI 2012).Nowadays, numerous companies have developed and customised an individual LPS to their specific requirements (Dombrowski and Mielke 2015;VDI 2012).
Industrie 4.0 describes the intelligent networking of people, machines and processes in industrial production by means of information and communications technology -often also referred to as the digital transformation of the manufacturing industry (Neugebauer 2019;Kagermann and Wahlster 2022).Industrie 4.0 holds various promises for productivity gains in LPS, ranging from high-percentage improvement rates in product quality, service quality, and flexibility to increased employee and customer satisfaction rates (VDI 2012;Bauernhansl 2017;Richter 2021;Kagermann and Wahlster 2022).There seem to be interdependencies between and an influence by Industrie 4.0 on the evolutionary development of Lean Production Systems (Dombrowski, Richter, and Krenkel 2017;Langlotz and Aurich 2021;Wagner, Herrmann, and Thiede 2017;Schumacher et al. 2021b).The general problem is uncertainty for both academia and practice about the changes in LPS design induced by digital transformation.
The design of LPS is usually performed by industrial engineering or operational excellence units, the target group for the applied research in this paper.The specific problem is that industrial practitioners are not always able to incorporate the latest trends into their operational practice and could benefit from suitable assistance by research, e.g. with process guides, maturity models, and research frameworks.Therefore, in this paper, we address the following research questions for the design of Lean Production Systems 4.0: (1) Which Lean Production System levels are most influenced by the digital transformation according to academia (and practice)?(2) What are the requirements for the design of Lean Production Systems 4.0 by industrial practitioners?(3) What are the guidelines for the design of future Lean Production Systems 4.0?
The aim of this paper is to present the findings on research questions (1)-(3) from our original research on the design of Lean Production Systems 4.0.For this, a systematic literature review on LPS level changes will be presented as well as the results of a field study with the important target group of industrial practitioners.In a synthesis of the academic and practical studies, we conclude 10 guidelines for the design of Lean Production Systems 4.0.
This paper is structured as follows.Section 2 gives an overview of relevant terminology and recent research contributions on the design of LPS 4.0.In Section 3, a systematic literature review with an analysis of LPS levels is presented.Section 4 encompasses the field study with industrial practitioners, composed of a two-staged research design of web survey and expert interviews.In Section 5, we discuss our findings in comparison to other academic contributors and with regard to our own research questions (1)-(3).Section 6 comprises concluding remarks and an outlook on further research streams.The structure of this paper with interrelations between sections, study stages and their respective results is visualised in Figure 1.

Related works and terminology
In this section, relevant terminology and research contributions on the design of Lean Production Systems 4.0 (LPS 4.0) are presented.The abovementioned definition by VDI ( 2012) serves as the central reference throughout this paper.Accordingly, Lean Production Systems not only encompass aspects of technical manufacturing processes but also organisational elements of planning and control of production processes in an overall company context.The general structure of Lean Production Systems is set up with a composition of the elements objectives, processes, principles, methods and tools, while the content of these elements is company-specific.Figure 2 gives an overview of the five main levels, building the organisational structure of a LPS.
In continuation of their contributions to the definition of Lean Production Systems in VDI (2012), Dombrowski, Richter, and Krenkel (2017) have systematically studied the interdependencies between LPS and Industrie 4.0, pointing out the driving forces Industrie 4.0 holds for the design of LPS.Based on this, Dombrowski and Richter (2018) propose a conceptual framework for Lean Production Systems 4.0, in which data management plays an important role as an accompanying process and the set of methods and tools is expanded by digital solutions (cf.Dombrowski et al. 2018).However, the details and  (2021) defines Lean Production Systems 4.0 as follows: 'A Lean Production System 4.0 represents a synergetic approach based on LPS and Industrie 4.0 to improve enterprise processes, which enables the implementation of Industrie 4.0 on the basis of a LPS in order to improve individual enterprise objectives' (Richter 2021).
Recent scientific literature provides multiple valuable reviews and studies on the combination of Lean Production and Industrie 4.0.Kolberg and Zühlke (2015) call for a suitable framework for the integration of Industrie 4.0, respectively cyber-physical systems into Lean Production Systems.Their work was continued with research contributions on cyber-physical production systems (CPPS) in a lead project on smart LPS funded by the German Research Foundation (Langlotz and Aurich 2021;Langlotz, Siedler, and Aurich 2021;Siedler, Langlotz, andAurich 2020, 2019).Performing a systematic literature review on Lean Thinking and Industrie 4.0, Bittencourt, Alves, and Leão (2019) contribute a rather qualitative discussion of relevant articles.Another review is presented by Sony (2018) with a qualitative discussion of mainly managerial implications.
Although enterprise-specific implementations are inherent to all LPS, studies systematically analysing industrial practice are relatively scarce.In a study on the changes of LPS induced by Industrie 4.0, Metternich, Meudt, and Hartmann (2018) present findings of workshops with 27 participating companies from German manufacturing industries, mainly focusing on the identification of design recommendations, best practices, and digitally advanced tools such as the value stream method 4.0 (cf.Meudt, Metternich, and Abele 2017).In a mixed quantitative and qualitative study with 74 manufacturing companies, Lorenz et al. (2019) empirically analyse the Lean and digitalisation maturity levels and conclude that Lean and digitalisation complement each other symbiotically for the benefit of operational performance measures (Lorenz et al. 2019).Saabye, Kristensen, and Waehrens (2022) carried out a single-company longitudinal embedded case study based on an action research and action learning approach, proposing organisational measures for manufacturing organisation to develop learning-to-learn capabilities (Saabye, Kristensen, and Waehrens 2022).
Several publications provide case studies or application cases, including industrial practice.Following the guideline of VDI (2013), Lean Production Systems comprise a set of 35 standardised methods with even more attributed tools for industrial engineering practice.In engineering research, there is broad consent about the evolution of LPS methods and tools towards new and advanced digital solutions (Höllthaler, Braunreuther, andReinhart 2018, 2019).In a general feasibility analysis, Mayr et al. (2018) give eight examples and one industrial use case for the conceptual conjunction of Industrie 4.0 tools and Lean methods, e.g.just-in-time/just-in-sequence 4.0, total productive maintenance 4.0, and visual management 4.0.With a systematic development of Jidoka 4.0, Deuse et al. (2020) combine an existing Lean method with Industrie 4.0 technology to the benefit of enhanced method reliability and performance.In addition to this, the potentials of Jidoka 4.0 systems for human-centred automation systems are discussed in detail by Romero et al. (2019).Wagner, Herrmann, and Thiede (2018) evaluate the results of a value stream design project with an industrial manufacturer and qualitatively conclude that Industrie 4.0 technologies can enhance the LPS efficiency.The abovementioned value stream mapping 4.0 has been practically evaluated in a learning factory environment and promises significant improvement potentials for industrial practice (Meudt, Metternich, and Abele 2017).Mrugalska and Wyrwicka (2017) briefly discuss three case studies for the enhancement of Jidoka and Kanban with Industrie 4.0 technologies.Burggräf et al. (2020) suggest the digital enhancement of continuous improvement practices by their framework for Kaizen 4.0.
For consistent use of nomenclature, we will now determine definitions of relevant terminology used throughout this paper.Industrial engineering is a discipline for the design, planning and optimisation of industrial value creation processes based on the use of methods from engineering science (Stowasser 2010;Bokranz and Landau 2012;REFA-Institut e.V. 2016).The term 'toolbox' encompasses the collection and description of a set of methods and tools -either physical or digital -for use in industrial engineering practice (VDI 2012).A tool is a standardised, physically or digitally available means that is necessary for the implementation of methods and serves to achieve business objectives (VDI 2012).A use case describes the exemplary application of a technology, method or tool.Use cases are implementation-oriented and provide technical solutions to specific problems.Use cases can be attributed with contextual information such as roles, business practices and relevant key performance indicators (DIN/DKE 2018).

Systematic literature review
A systematic literature review (SLR) is a conclusive, systematic, and reproducible method for the identification, assessment, and synthesis of existing research contributions (Fink 2010).The method focuses on a specific thematic domain, for which the reviewers need to examine the state of the research beforehand.The literature review itself is a process of secondary analysis of already existing research contributions (Torraco 2016).Based on Fink (2010), the methodological approach for this SLR is divided into six phases: (1) definition of a research hypothesis, (2) identification of relevant and appropriate data bases, (3) postulation of suitable search terms per data base, (4) definition of criteria for the screening process of searched results, (5) execution and documentation of the literature review by reviewers, (6) synthesis of the SLR results (Fink 2010).
With the relevant contributions on LPS 4.0 at hand, the next step in our research is a systematic literature review of the impact of the digital transformation on each level of LPS by measuring the number of contributions.This approach corresponds with the overall research questions and adds quantified observations from recent literature.More specific, the aim is to characterise and analyse recent literature regarding the impact of Industrie 4.0 on Lean Production System levels, following the definition of VDI (2012).As Figure 2 shows, these system levels are (1) objectives, (2) enterprise processes, (3) principles, (4) methods, and (5) tools (VDI 2012).The results of this SLR were recently published in Schumacher et al. (2021b).
As an important detail for this SLR, it is inherent to LPS that the items of each level are interrelated with each other.While objectives, processes, and principles are distinctly delineated levels, the literature is rather inconsistent regarding methods and tools.Methods and tools can be related to one or more principles and serve as the operational measures for the realisation of objectives.Both methods and tools can directly be linked with a principle (Schmidt 2011).Usually, one or several tools are used within a method to contribute to a principle.Hence, a logical separation into two levels of LPS is mostly common in literature.See Schumacher et al. (2021b) for a detailed discussion of these interrelations.Figure 3 provides definitions for principles, methods, and tools, as well as an illustration of possible combinations of these elements.
The SLR was carried out according to the presented approach by Fink (2010)  The following screening criteria were applied to the three databases.Only literature from 2017 or later was considered for the SLR in order to analyse the latest contributions with direct relation to Industrie 4.0.World Wide Science and Scopus allowed for the exclusion of irrelevant thematic fields, e.g.'math', 'health', 'care', 'social', and 'diet'.For methodological quality screening reasons according to Fink (2010), the papers included were required to be peer-reviewed and with at least one citation, when older than 12 months.Duplicates were filtered out during title, abstract, and full text screening.Initially, 1620 records were identified.After screening, 62 records were qualified for the SLR (Schumacher et al. 2021b).Figure 4 illustrates the screening process with the numbers of papers per database for each stage of the screening.
The core results of the SLR are shown in Table 1 with the literature being classified according to their contribution to LPS level.The table indicates whether a publication describes an influence of digital transformation on the LPS level (x) or does not show a clear position (blank).Hence, one paper can be classified supporting none, one, or many LPS levels to be influenced by digital transformation.A more comprehensive table with a summary for each of the 62 analysed papers is made available online as extended material by Schumacher et al. (2021c).
The analysis of the review shows a clear overall result of the assessment on different levels, based on recent academic contributions only.All 62 papers address the influence of the digital transformation on tools in LPS.With a significant gap, not methods but processes are mentioned second most with 39 out of 62 papers.Figure 5 gives an overview of the absolute and relative contributions per LPS level.The literature review suggests that the digital transformation would have the most significant impact on tools and processes, while methods and principles might be influenced indirectly by digital tools (cf.Schmidt 2011, Figure 3).Despite these significant empirical findings from academic literature, the SLR results remain to be challenged with implications by LPS practice in industry.

Field study on Lean Production Systems 4.0
Scientific literature on the recent developments in the design of Lean Production Systems provides a set of hypotheses which can only be investigated by adding industrial practice to the analysis.Therefore, we conducted a field study surveying practical users from leading German manufacturing companies.This study puts the predominantly scientific-theoretical analyses into an The findings of our field study are threefold.First, the results of the web survey with a large sample size of industrial practitioners help to evaluate scientific hypotheses.Second, detailed interviews with leading experts lead to insights on existing and upcoming design requirements and paradigms for LPS.Third, we present the synthesis of both study stages and conclude with guidelines for the design of Lean Production Systems 4.0 as the central result of the field study.

Research methodology and study design
The central research question of the field study is concerned with the identification of requirements for the design of Lean Production Systems 4.0.The research question encompasses the related work by various scientific contributions presented in Section 2 and was subdivided into seven hypotheses.Table 3 gives references for the research question and each of the seven hypotheses.The hypotheses will be analysed in detail in Section 4.3.
The methodical operationalisation of the hypotheses is a crucial scientific process for valid study results.For this, the study was designed with two analytical stages.The first stage was a quantitative analysis of 73 industrial practitioners from German manufacturing companies with high knowledge in Lean Production and Industrie 4.0.The anonymous participation in the iteratively pretested questionnaire was realised in form of a web survey.Each hypothesis was subdivided into relevant questions according to good methodical practice, with a set of questions ensuring the cross-checking for analytical purposes.The results for each question were then analysed in order to evaluate the approval or dismissal of the according hypothesis.All hypotheses and corresponding questions are listed in Table 4.
The second stage of the study was a mainly qualitative analysis of expert interviews with 18 German manufacturing companies.Due to the corona pandemic,    (2022).In total, 336 codes were attributed to statements made by the interviewees.Using inductive category formation for identification of sub-categories and categories, 119 codes were analysed quantitatively and 18 codes were analysed qualitatively.
The study was conducted in the period from June 2020 until February 2021.The German companies participating in both stages of the study mainly belong to automotive and mechanical engineering industries.

Statistical bias and reliability of the web survey
To assess statistical bias, survey results were tested for non-response bias and common method bias in accordance with Tortorella et al. (2020c).For non-response bias, Mann-Whitney U test based on the nonparametric, Likert-scaled items of the web survey was performed to compare differences between early and late responses (Armstrong and Overton 1977).At a significance level of α = 5%, no significant differences (p < 0.05) in central tendency (median) were identified between early and late responses.To test for common method bias, Harman's single factor test was conducted.Although this test has several limitations, it is widely used in the scientific literature.The test results in a value of 18.25%, which indicates that common method bias is not likely to be a problem for the results of the web survey (Podsakoff et al. 2003; Aguirre-Urreta and Hu 2019).
The core of the web survey consists of four blocks of questions containing between three and five individual items.To assess the reliability of the questionnaire, Cronbach's alpha was determined for each block of questions.Cronbach's alpha ranges from 0.655-0.809and thus is at least acceptable for each question block (Vaske, Beaman, and Sponarski 2017).The individual values of Cronbach's alpha per question block are incorporated in Table 5.From a statistical point of view, the questionnaire can be considered reliable.

Results of the web survey
The web survey was carried out in summer 2020 with 75 participants.Due to the elimination of two blank entries, the net sample size is 73 participants.The non-random sampling process and sample size do not qualify for representative assumptions to be made for whole industries and other economies.However, the group of participants provides valuable insights into industrial practice, which have not been generated before.The questionnaire used in the web survey comprises 26 questions, with the first five questions surveying personal and company-related data of the participants.Table 4 gives an overview of all 21 questions with regard to the hypotheses.Asked about their knowledge with respect to Lean and Industrie 4.0, both categories were answered with 'high' or 'very high' by over two-thirds of the participants, stating the proficiency of the sample group.
Each of the 26 questions was statistically and graphically analysed in detail.Most of the questions were operationalised by a five-level Likert scale from 'strongly disagree' to 'strongly agree,' while hypothesis 2 was surveyed in a scenario selection and hypothesis 6 in a ranking.In addition to this, when asking for details with regard to changing LPS elements, participants were facilitated with an option to submit individual textual inputs.
As a starting point of our study, hypotheses 1 and 2 are directed towards the interdependencies of Industrie 4.0 and LPS in general.While hypothesis 1 implies dependencies between LPS design and Industrie 4.0, hypothesis 2 builds upon three scenarios for the future design of LPS integrating or separating Industrie 4.0.Hypothesis 1 finds broad support by industrial practice.The weakest interrelation of Lean being beneficial for I4.0 implementation in question 6 shows no opposition and strong approval by the participants.Questions 7 and 8 -checking the necessity of Lean as a basis for Industrie 4.0 in two complementing formulations -come with a few opposing opinions, but still underline the broad consent of Lean being a necessary and beneficial basis for Industrie 4.0, which can be assumed true for the sample group of the study.Moreover, hypothesis 2 confronts the participants with three scenarios for the future design of LPS -a strictly sequential implementation of first Lean and then I4.0 (scenario 1), simultaneous but strictly separate implementation of Lean and I4.0 (scenario 2), or an integrated approach for the combined implementation of Lean and I4.0 (scenario 3).As common sense in research suggests, with 82% the vast majority of participants agreed to scenario 3, an integrated approach for Lean and I4.0.
Hypothesis 3, 4 and 5 are set up for a more detailed analysis with regard to the influence of Industrie 4.0 on the five levels of LPS according to VDI (2012): objectives, processes, principles, methods, tools.Hypothesis 3 states the general influence of I4.0 on all LPS levels.With approval rates from 64 to 89%, hypothesis 3 can be assumed to be valid.Hypothesis 4 implies that I4.0 will lead to new elements emerging in each of the LPS levels.After analysing the results for each LPS level, this hypothesis also proves to be accepted by the participants of the study, with little less approval rates between 58% and 84%.Hypothesis 5 complements the previous hypothesis by assuming that I4.0 would cause existing  (Schumacher, Bildstein, and Bauernhansl 2020;VDI 2012;cf. Dombrowski and Mielke 2015).Hence, these levels seem to be the most influenced by Industrie 4.0.The participants were asked to rank the influence of Industrie 4.0 on the five LPS levels in a strict hierarchy of five ranks.Hypothesis 6 was approved by the participants, with 'tools' leading the ranking by far, followed by 'methods' with a clear distance to the third rank.Assumedly because of the different type of the question in form of a ranking, the participation rate for this question was noticeably lower than for any other question (n = 54).
Hypothesis 7 was used to concretise the analysis for a suitable framework for future LPS structures.Most of the scientific literature describes evolutionary developments of existing LPS structures, while some literature calls for drastically new approaches.For this, Hypothesis 7 is seeking answers on the suitability of the LPS structure for the design of future production systems.
In addition to descriptive analysis, statistical hypothesis testing was performed for hypotheses 1, 3, 4, 5 and 7.For this, the 5-point Likert-scaled data was transformed into numeric values (from 1 ∧ = 'strongly disagree' to 5 ∧ = 'strongly agree') and regarded as metric scaled.The answers were considered to be normally distributed.The null hypothesis states that the population does not agree with the analysed hypothesis (population mean < = 3).The alternative hypothesis is characterised by a population mean above 3.The described assumptions and contextual prerequisites were operationalised by a one-sided t-test.For this sample, the t-tests show that hypotheses 1, 3, 4, and 7 can be accepted at a significance level of α = 5% (p < 0.05).With three non-significant single statements, hypothesis 5 has to be rejected.Hypotheses 2 and 6 were operationalised in a different manner with clear results and thus were excluded for statistical hypothesis testing.See Table 5 for statistical data from hypothesis testing.
In summary, six out of seven hypotheses from research have been found supported by industrial practice.Lean Production is a necessary basis for the design of production systems with Industrie 4.0 technologies.LPS should be designed in an integrated approach, including Industrie 4.0.All LPS levels are subject to digital transformation, with methods and tools being the most influenced.The LPS structure is approved for the design of current and future production systems.
The sample group of participants in this web survey is higher than comparable practical studies on LPS 4.0 (cf.Dombrowski and Richter 2018;Metternich, Meudt, and Hartmann 2018;Dillinger, Formann, and Reinhart 2020) and provides valuable insights into leading German manufacturing companies on yet solely scientifically discussed theories.However, the results do not qualify for statistically representative conclusions.For at least one hypothesis, a more detailed analysis is needed in order to obtain a thorough impression of industrial practice.

Results of the expert interviews
The expert interviews were used to add mainly qualitative detail to the analysis.In 18 interviews with leading practitioners from German manufacturing companies, the existing structures of LPS as well as the trends and resulting requirements for industrial engineering practice were studied.
The first segment of the interviews gave a clear impression on the current state of LPS design in industrial practice.Most of the companies had already implemented and revised their company-specific LPS in the 1990s and 2000s.Many experts reported that first attempts in solely copying the TPS elements were not sufficient and had to be reworked in order to meet company-specific requirements and cultural prerequisites.12 out of 15 companies consider their LPS as fully implemented.When asked for LPS principles, methods and tools in use, the experts named typical Lean elements from corresponding catalogues such as VDI ( 2013).The most named principles are continuous improvement process (9 out of 15), zero defects ( 6), elimination of waste ( 5), pull principle (4), push principle (4).Methods named comprise Kanban (7), value stream design (6), Hoshin Kanri (4), TPM (3), Poka Yoke (3).Especially large enterprises are already actively enhancing their portfolio of methods and tools by digital solutions.In addition to this, the organisational structure and strategic implementation were surveyed.
As a transition between the two segments, the experts were asked for their current LPS project activities and the associated individual challenges.While being involved in day-to-day LPS activities as well as long-term initiatives such as MES implementations, the experts shed light on relevant upcoming issues for LPS design.Flexibility is mentioned by 10 experts as a central challenge, with further corresponding challenges such as complexity (7), standardisation (5), and volatility (4).Industrial production systems need to offer increased flexibility to meet short-cycled demands by volatile markets.Along with flexibility, the experts stress the importance of transparency as an overall principle and data management as well as data analytics for the design of LPS.According to the experts, successful data management can only be obtained by covering data quality and consistency, data usage, and according data processes.In addition to these internal challenges, external influences are dominated by customer-driven product individualisation with 11 statements by experts.
In the second segment with 18 experts interviewed, two closed questions were asked in order to obtain a clear impression of the interviewees' positions.Challenging the fit of LPS for the design of future production systems, the experts were asked if the VDI (2012) remains valid for future LPS design.15 out of 18 experts agreed to the statement that this quasi-standard remains a suitable basis for the design of future production systems.In search of the transformational process, the experts were asked whether the coming approach for the design of production systems would be evolutionary development or revolutionary innovation.In compliance with the first question, all but one expert see an evolutionary development of existing LPS structures enriched by Industrie 4.0 solutions.
Continuing the qualitative analysis of the second segment, the experts were asked for further changing elements in the design of LPS.Process environments were described as becoming increasingly interconnected within production networks, based on end-to-end processes requiring intra-and inter-company cooperation for seamless production systems.Furthermore, six experts addressed the decline of the flow principle and two experts the decline of the pull principle for the benefit of more flexible and changeable structures.
Due to its direct impact on productivity measures, the industrial engineering of methods and tools according to principles is the focal point of this field study.Throughout the interviews, the requirements of the industrial practitioners for the design of future LPS were collected, attributed with codes, summarised and clustered.The experts confirm typical requirements such as the use of consistent standards and regular auditing of LPS performance throughout the company.As with other business cases, digital solutions only tend to be pursued if they outperform existing methods or tools and maintain costefficiency.A methodical toolbox covering LPS methods and tools could serve as a central conceptual component.Some of the experts imply to already use a form of toolbox, while it remains unclear to which extend.Most descriptions involve a methods catalogue provided via corporate intranet or content management systems.Concretely asked, 14 out of 15 experts consider a LPS toolbox to be an important instrument for industrial engineering practice.The most relevant requirements from the qualitative analysis are presented in Table 6, involving Industrie 4.0 use cases and according systematic roll-out processes as main categories.
With broad consent among experts, the established framework for Lean Production Systems remains central for industrial practice and will be evolutionary enhanced by Industrie 4.0 technologies.The success of LPS in manufacturing companies is highly dependent on the cultural integration and involvement of the workforce.With Industrie 4.0, the definition of roles and processes for data management becomes crucial in LPS.Industrial engineering is facing increased complexity of newly introduced digital methods and tools, which should be Table 6.Qualitative requirements from industrial engineering for future LPS design.

Standardisation and application
-Standardised description and systematic documentation of methods and tools -Scouting of new technologies, methods and digital tools -Regular audits and updates of existing methods and tools -Reasonable efforts for maintenance of methods and tools -Simple and consistent structure for the applicability of methods and tools -Simplification of complex LPS structures Toolbox -Toolbox as an important instrument for industrial engineering -Toolbox as an aid for the management of increasing complexity in production -Logical superstructure should be covered in a generic framework -Toolbox needs a systematic scheme for individual adaptions and application processes -Modular design with methods and tools -Ease of use and training compatibility -Items and solution sets need to be available in a web-based application Use cases -Methods and tools need to be linked with concrete use cases -Digital use cases need to be qualified in a pilot phase -Digital use cases need to be rolled out throughout the whole company -Scalability is a selection criterion for Industrie 4.0 use cases -Use cases should be assigned with a problem and solution component Roll-out -Roll-out process should comprise qualification and establishment of methods and tools -Regular exchange and interdisciplinary teams -Adjustable for individual Lean and I4.0 maturity levels of different business areas -Incorporation of resilient and stable processes during roll-out phase -Methodical support of roll-outs systematically organised, qualified and rolled out by means of a LPS toolbox.

Guidelines for the design of Lean Production Systems 4.0
As a synthesis of the insights provided by this study, we have developed 10 guidelines for the design of Lean Production Systems 4.0.These guidelines are based on the systematically identified requirements of industrial practice for the design of Lean Production Systems.Each guideline was validated by the data collected throughout the study and is related to a validated hypothesis as well as several codes from the analysis of the expert interviews.The formulation of design guidelines follows the five systematic guidelines for the methodologically correct creation of design guidelines, according to Erlach et al. (2020).Table 7 defines and explains the guidelines in detail.Figure 6 illustrates the relations between the guidelines and the levels of Lean Production Systems.
The guidelines for the design of Lean Production Systems 4.0 are intended for industrial engineering practices in manufacturing companies.They are based on the empirical database of the study and reflect the state of knowledge on the current status of production systems as well as relevant technological and overall system developments in the digital transformation of production.

Limitations of the field study
The presented field study is not free of limitations.First of all, the results of the study cannot simply be projected onto any international manufacturing company and economy.Statistically, the sampling process and sample size of the study do not allow for all universal conclusions, but the data proved to be reliable and free of biases.As a strength and weakness at the same time, the framework for our research is given by the German quasi-standard for Lean Production Systems, VDI (2012), which is then combined with the mainly Germancoined term Industrie 4.0 and its related works.Even more, we have studied only German manufacturing companies.It is clear that this can have an impact on the results of our study and that the results cannot simply be transferred to other economies and domains.Solely English literature such as Bittencourt, Alves, and Leão (2019) emphasise the importance of German literature in this field to be added to the international discussion.In return, we consider ourselves to be aware of relevant international research streams on Lean Production Systems 4.0, as well.
As a result, our study remains a valuable and detailed analysis of German Lean Production Systems and their design but needs to be interpreted considering the accompanying circumstances.We took measures to guarantee a high level of knowledge by participants represented in the web survey and carefully set up a representative group of industrial practitioners for the expert interviews.All interviewees are directly involved in the LPS design of their respective producing company.To obtain a thorough impression, not only large enterprises but also small and medium-sized enterprises as well as technology providers as an innovative group were involved in the study.In addition, the study results were also challenged and confirmed by a larger industrial working group on Lean Production Systems 4.0, consisting of over 30 experts from 15 producing companies from Germany, Austria, and Switzerland (cf.Schumacher et al. 2022).We encourage the conduction of further studies with even higher sample sizes and more diverse participants.Moreover, the study motivates several additional research questions with greater detail to be analysed.

Discussion
In this section, the results of the analysis of academic literature as well as the field study are discussed with regard to related other literature in Section 5.1 and the initially stated three research questions in Section 5.2.

Discussion of related literature
With regard to the systematic literature review from Section 2, other SLRs in the field of LPS 4.0 should be addressed.In a SLR on Lean Production in complex socio-technical systems, Soliman and Saurin (2017) conclude that the contribution of Lean Production to manage complexity is a future research field which could be supported by a suitable implementation framework, as was also found in our study.Buer, Strandhagen, and Chan (2018b) performed and earlier and insightful SLR with a lower number of contributions, giving insights on the strong link between Industrie 4.0 and Lean Production.In Buer et al. (2021), the complementary effect of Industrie 4.0 and Lean is given evidence by correlating data of operational performance measures.As one of the results of a bibliometric literature analysis, Ejsmont et al. (2020) find various 'Lean Industry 4.0 advantages' based on Industrie 4.0 technologies supporting Lean Production measuring factors.There are valuable recent approaches for the integration of digital measures from Industrie 4.0 in Lean Production Systems such as Moeuf et al. (2018), Mittal et al. (2020) and Rosin et al. (2020), supporting the approved hypotheses of our field study as well as the guidelines for LPS 4. Case studies are another practical approach to analyse LPS 4.0 developments.E.g.Ciano et al. (2021) analyse one-to-one relationships of Lean Production techniques and Industrie 4.0 technologies across a set of eight cases from manufacturing firms, stressing the importance of Table 7. Guidelines for the design of Lean Production Systems 4.0 (Schumacher et al. 2021a).

Guideline Description
(1) People as central success factor The design of Lean Production Systems is to be oriented towards individual human needs and the participation of interdisciplinary teams.
Processes and technologies are to be designed in a human-centred way.
Employees must be actively involved in decision-making processes.Participation of experienced employees leads to innovation and avoidance of waste.Through communication and training, the philosophy of a learning organisation becomes part of the corporate culture.(2) Integrated approach of Lean and Industrie 4.0 The design of Lean Production Systems is to be implemented in an integrated approach of Lean Production and Industrie 4.0.
Lean Production Systems continue to develop in an evolutionary manner.
Process structures and Lean methods are a necessary basis for Industrie 4.0 implementations.The realisation in an integrated approach enables networking and data-based optimisation potentials in industrial engineering.
(3) Customer integration and individualisation The design of Lean Production Systems must be oriented towards customer benefits through elements of customer integration and individualisation.
Lean Production Systems must not only be used for internal production optimisation, but must also further integrate customers into processes.Short innovation cycles and volatile markets give rise to individualisation requirements on the customer side.In terms of production systems, customer individualisation means an increased importance of responsiveness, flexibility and adaptability.(4) Inter-company cooperation and end-to-end processes The design of Lean Production Systems is to be extended to inter-company cooperation for a continuous consideration of end-to-end processes without system discontinuities.
Lean Production Systems do not only concern peripheral areas of a single company, but increasingly require an extension of the system boundaries beyond company borders.The horizontal integration of suppliers and customers must also be reflected in the design of LPS.The consideration of personalised and automated end-to-end processes without system breaks promises great optimisation potential.( 5) Data management and transparency The design of Lean Production Systems must take into account systematic data management and system-wide transparency.
Data management includes the design of roles and processes in data collection, data continuity and data processing.Transparency is an important Lean principle and an enabler for data-based optimisation and autonomy in production systems.The range of tasks in industrial engineering is being expanded to include data analytics.(6) Flexibility and changeability The design of Lean Production Systems must be used consistently to support flexibility and changeability.
Through increased networking and near-real-time planning capabilities, flexible and changeable structures can be further implemented in production systems.In the future, the pull and flow principle can be displaced in some areas and assembly lines can be substituted by fluid production systems.(7) Standardisation with necessary degrees of freedom The design of Lean Production Systems must allow for systematic standardisation while maintaining degrees of freedom for individual application domains.
Industrial engineering defines company standards in LPS with principles, methods and tools.The individual adaptation of the standards to the application domain must be considered.Without appropriate degrees of freedom, the successful implementation of a LPS in different economic zones, plants, business units or value streams cannot succeed.(8) New digital methods and tools The design of Lean Production Systems must be carried out with the integration of new digital methods and tools.
Established methods and tools from Lean Production Systems are supplemented, expanded and partly replaced by Industrie 4.0.Industrie 4.0 increases the complexity of the available technical and methodical means.In industrial engineering, new competencies in technology management and data-based optimisation occur.New digital solutions must prove cost-beneficial to be applied.(9) Toolbox for methods and tools The design of Lean Production Systems is to be accompanied by a toolbox for methods and tools.
A toolbox for methods and tools in Lean Production Systems is to be used for documentation and cataloguing in directories or databases.It needs a logical systematic with a modular structure and must include a roll-out process for qualification and company-wide implementation.The toolbox for industrial engineering must be made easily accessible and intuitive for users.(10) Industrie 4.0 through use cases The design of Lean Production Systems must be implemented with the integration of new technological Industrie 4.0 applications and their description in the form of use cases.
New Industrie 4.0 technologies can be described in the form of use cases and thus be used for qualitative and quantitative assessments of the implementation success.Use cases are to be used as comparable application examples with a problem and solution component.
practical approaches to add detailed evidence to foremost solely theoretical analyses.This seems to be reasonable for both the presented case study approach and our practical field study.In contrast to the abovementioned cross-sectional studies, the longitudinal case study by Saabye, Kristensen, and Waehrens (2022) provides a highly practical intra-company study on how to establish learning-to-learn capabilities in a Lean and Industrie 4.0 setting, with the potential to complement our field study and the resulting design principles.

Discussion of research questions
In this section the contribution of our study to the research questions is reflected.
(1) Which Lean Production System levels are most influenced by the digital transformation according to academia (and practice)?
The digital transformation of LPS is a consensus in academia and practice (cf.Ciano et al. 2021;Buer et al. 2021;Kamble, Gunasekaran, and Dhone 2020).This research question is set up to identify differences with regard to the LPS levels, which has not been done before.
When analysing, it is important to acknowledge that the assessment of the digital transformation on LPS levels is highly dependent on the professional qualification and background of the assessors as well as the research field of the journals analysed.With respect to these biases involved, the combined results of the presented analyses provide important insights.First, tools are attributed with the highest impact by Industrie 4.0, especially through new technologies evolving.Methods are directly influenced by new digital tools and need to be constantly updated.In line with the close relationship between tools and methods presented in Section 3, this seems conclusive.Second, enterprise processes are highly dynamic and subject to multiple changes related to Industrie 4.0.Third, objectives and principles seem to be relatively stable in comparison to other LPS levels.
(2) What are the requirements for the design of Lean Production Systems 4.0 by industrial practitioners?
The field study gives new and unique insights into the requirements by a large number of industrial practitioners for the design of LPS 4.0.Described in detail in Section 4.4, industrial engineers are facing increased complexity in the management of existing methods and new tools.A described solution could be the systematic development of a corresponding LPS 4.0 toolbox.Data management and analytics enhance the individually required skillset as well as the need for suitable infrastructure and corporate collaborative efforts.When asked for the framework of future LPS, the experts agreed on an evolutionary development of existing structures as prevalent in VDI (2012), rather than a completely new framework.
(3) What are the guidelines for the design of future Lean Production Systems 4.0?
As a synergy of the theoretical background and the practical inputs by industry, 10 guidelines for the design of LPS 4.0 were developed in Section 4.5, giving the answer to this research question.Since guidelines are only recommendations for specific implementations, it is important to further operationalise these guidelines for practical use in industry.

Conclusions
This paper addresses the highly dynamic applied research field of Lean Production Systems 4.0, especially their design by industrial engineers.For this, academia and industrial practice were analysed in a combined approach with a high amount of scientific literature analysed and a unique expert group of industrial practitioners surveyed.
The systematic literature review of 62 out of 1600 scientific papers shows that especially the tools, processes, and methods in Lean Production System are subject to digital transformation.By involving industrial practice in a field study, quantitative and qualitative insights were used to identify requirements and guidelines for the design of future Lean Production Systems 4.0.The theoretical and practical contributions, as well as the limitations and future research, are presented in the following subsections.

Contributions to theory
Engineering research is concerned with the design of Lean Production Systems 4.0.Our paper adds valuable insights for academics.First, the SLR results provide researchers with a structured in-depth analysis of changes in LPS levels.In detail, 62 recent research contributions were analysed and contextualised.Overall, the results show that tools and processes are highly influenced by Industrie 4.0.Second, the combination of the literature review with a practical field study adds detail to our findings.LPS are implemented in industrial companies and thus must be analysed from this perspective, as well.The twostaged study design provides research with quantitative and qualitative insights of industrial engineering.The identified requirements by industrial practice should be considered in future theoretical research activities.
Third, the guidelines for future LPS 4.0 design are the synergetic result of this paper.Academia should discuss and challenge these guidelines in detail.Furthermore, the guidelines should be incorporated into further research activities.
In summary, all three research questions were answered throughout the paper.Even more, the paper presents a multitude of valuable quantitative and qualitative insights from industrial engineering practice.

Contributions to practice
Except for single case studies mentioned in Section 2, we are not aware of any study on LPS 4.0 involving a high number of industry practitioners to evaluate research hypotheses in-depth.Hence, our study contributes with an original as well as an applied research approach.The threefold results of the web survey, expert interviews, and synthesis provide new and valuable insights for the design of LPS 4.0.In addition to empirical data, the field study provides practitioners with relevant qualitative details from their peer group.Important research fields derived from the field study are the focus on humans, data management, flexibility and changeability, and intercompany cooperation, which must be integrated into LPS 4.0 practice.For industrial engineering, a structured approach for the evolution towards digital methods and tools is needed, which is described with detailed requirements for a corresponding toolbox in this paper.
We encourage further studies involving industrial practice and plan to conduct a longitudinal study with the application of a LPS 4.0 toolbox based on our proposed research framework.

Limitations and future research
It is important to address the underlying limitations of the research methods and results presented in this paper.The systematic literature review is the most structured and methodical way of analysing academic contributions to a thematic field.However, the screening and analysis processes are both dependent on the individual decision making of the reviewers and it is always possible that valuable contributions are filtered out due to missing the requirements of a certain criterion.The field study is mainly limited with regard to statistical representation and a partially primed perspective from German and English engineering literature and German manufacturing companies analysed.A detailed discussion of the limitations of the field study is presented in Section 4.6.
With regard to Lean Production Systems, there are several conceptual proposals for frameworks integrating Industrie 4.0 (Dombrowski et al. 2021;Dombrowski and Richter 2018;Gregori et al. 2017;Richter 2021).However, these approaches either do not address the specific industrial engineering tasks of the design of LPS methods and tools or do not provide enough details to be considered for practical application.In future research, we will develop an according framework for methods and tools in Lean Production Systems 4.0.For this, we have identified three important modules for our research.
First, the theoretical and practical requirements of industrial engineering for a methodical framework need to be systematically identified and validated.Second, the framework needs to be conceptualised and developed, including a process model for the use and a described toolbox for industrial engineering.Third, it is crucial to uphold the applied research with practitioners involved.Therefore, a cross-sectional and longitudinal study (cf.Saabye, Kristensen, and Waehrens 2022) with experts from multiple producing companies with several iterations over one year is planned, in which the methodical research framework, as well as the integrated toolbox, will be co-developed and evaluated stepwise.Methodically, a Delphi study is planned to further analyse trajectories for LPS 4.0 developments.
for Energy Efficiency in Production EEP at the University of Stuttgart.Bauernhansl is on diverse advisory boards and management committees in industry, associations, research and politics.He is a member of WGP, the German Academic Society for Production Engineering, as well as being a member of the strategy committee of the German government's 'Platform Industrie 4.0' and Vice Chairman of the Steering Committee Alliance Industrie 4.0 BW.He has written and edited numerous books on topics such as adaptability in manufacturing, Industrie 4.0 and production management.

Figure 1 .
Figure 1.Overview of the paper structure.

Figure 3 .
Figure 3. Possible settings for the interrelation of methods and tools (Schmidt 2011).
in the time from April to June 2020.English and German search terms were applied to identify the most significant scientific contributions.The databases searched are Scopus, Web of Science, and World Wide Science.This is an example for a search term (TITLE-ABS-KEY) used in Scopus: ((Ganzheitlich * Produktionssystem * ) OR (holistic production system) OR (Lean Produ * tion) OR (Lean Produ * tion System) OR (Toyota Produ * tion * ystem)) AND ((Digital * Transformation * ) OR (Industr * 4.0) OR (Digitalisierung) OR (Digitali * ation) OR (Digital * Wandel)).

Figure 4 .
Figure 4. Number of papers per database throughout the screening process (Schumacher et al. 2021b).

Figure 5 .
Figure 5. Absolute and relative contribution rate of scientific literature regarding the Industrie 4.0 impact on each LPS level (Schumacher et al. 2021b; n = 62).
Q11: 'Industrie 4.0 leads to an extension of Lean Production Systems by new objectives.'Mode = 'agree' Median = 'agree' Approval rate: 67.1%; n = 73 Q14: 'Industrie 4.0 leads to an extension of Lean Production Systems by new processes.'Mode = 'agree' Median = 'agree' Approval rate: 68.5%; n = 73 Q17: 'Industrie 4.0 leads to an extension of Lean Production Systems by new principles.'Mode = 'agree' Median = 'agree' Approval rate: 57.7%; n = 71 Q20: 'Industrie 4.0 leads to an extension of Lean Production Systems by new methods.'Mode = 'agree' Median = 'agree' Approval rate: 76.4%; n = 72 Q23: 'Industrie 4.0 leads to an extension of Lean Production Systems by new tools.'Mode = 'strongly agree' Median = 'strongly agree'/ 'agree' Approval rate: 84.0%; n = 70 H5: Due to Industrie 4.0, existing elements of LPS are losing their relevance.Q12: 'Due to Industrie 4.0, existing objectives of LPS lose in relevance.'Mode = 'disagree' Median = 'disagree' Approval rate: 9.6%; n = 73 Q15: 'Due to Industrie 4.0, existing processes of LPS lose in relevance.'Mode = 'partially agree' Median = 'partially agree' Approval rate: 29.2%; n = 72 Q18: 'Due to Industrie 4.0, existing principles of LPS lose in relevance.'Mode = 'disagree' Median = 'disagree' Approval rate: 15.3%; n = 72 Q21: 'Due to Industrie 4.0, existing methods of LPS lose in relevance.'Mode = 'partially agree' Median = 'partially agree' Approval rate: 21.9%; n = 73 Q24: 'Due to Industrie 4.0, existing tools of LPS lose in relevance.'Mode = 'partially agree' Median = 'partially agree' Approval rate: 34.7%; n = 72 H6: Industrie 4.0 has the strongest influence on the LPS levels of methods and tools.Q25: 'Please rank the impact by Industrie 4.0 on LPS levels, from rank 1 (highest) to rank 5 (lowest).'Rank 1: 'tools' Rank 2: 'methods' (operationalised in a ranking; n = 54) H7: The LPS structure provides a suitable basis for the design of future production systems.Q26: 'If LPS levels are being redesigned according to Industrie 4.0 requirements, the LPS structure will remain relevant for the design of production systems.'Mode = 'strongly agree' Median = 'strongly agree' Approval rate: 88.9%; n = 72 executives and practitioners in the field of Lean Production Systems, industrial engineering and operational excellence.Leading German manufacturers were joined by representatives of small and medium-sized enterprises and by representatives from innovative technology providers.Half of the interviewed experts had also taken part in the web survey of the first stage of the study.The expert interviews were transcribed, attributed with systematic codes, and then analysed according to Mayring

Figure 6 .
Figure 6.Guidelines for the design of Lean Production Systems 4.0 (Schumacher et al. 2021a).
0 design.Similar analyses of the interrelation between Industrie 4.0 technologies and Lean Production are mentioned in Section 2. Multiple dissertations and research projects also aim to develop models or frameworks for the implementation of Industrie 4.0 into Lean Production Systems (Richter 2021; Nöhring 2021; Deuse et al. 2020; Langlotz and Aurich 2021).Studies including industrial practice such as the presented field study are relatively scarce.Comparable existing LPS 4.0 studies are based on smaller sample sizes (Dombrowski and Richter 2018, n = 13; Metternich, Meudt, and Hartmann 2018, n = 27; Dillinger, Formann, and Reinhart 2020, n = 30) In a mixed quantitative and qualitative study with 74 manufacturing companies, Lorenz et al. (2019, n = 74) empirically analyse the Lean and digitalisation maturity levels and conclude that Lean and digitalisation complement each other symbiotically for the benefit of operational performance measures (Lorenz et al. 2019).The abovementioned study by Buer et al. (2021, n = 75) surveys the operational performance of 75 manufacturing companies at the intersection of Lean Production and Industrie 4.0.With a different focus on Lean Automation, Rossini et al. (2022, n > 200) empirically validate the positive interrelation of Lean and Industrie 4.0 with a large data set of over 200 manufacturing firms.Especially with the mixed-method study design in two stages (web survey, n = 73; expert interviews, n = 18), our field study seems to add various new insights to academia by detailing the reality of industrial practitioners seeking for evolutionary development of their LPS 4.0.

Table 2 .
Methodical and temporal specifics of the study stages.

Table 3 .
Research question and hypotheses of the study.
view guideline was designed with mainly open questions and topics to encourage free contributions by the experts and their practice.The guideline and lead questions of the interview are listed in Table A1 in the Appendix.The experts represent the target group of

Table 4 .
Hypotheses, questions and descriptive statistics of the web survey.

Table 5 .
Statistical data from hypothesis testing.