Lean Manufacturing: A Bibliometric Analysis, 1970- 2020

The study aims to investigate lean manufacturing from a bibliometric perspective. A systematic analysis was performed on 1893 documents extracted from the Clarivate Analytics Web of Science (WoS) Collection under the “Lean Manufacturing” (LM) subject category between 1970 to 2020. The R programming is utilized for determining the evolution and progress of lean through the topmost contributing publications, authors, journals, and countries. The network representation is performed using VOS viewer, highlighting the focus areas of LM with co-occurrence, co-citation, and bibliographical coupling of the extracted data. The results indicate LM was initially focused predominantly on topics such as lean thinking and organizational performance by various authors. Later, the LM topic was integrated with six sigma, sustainability, and environmental assessments for overall process improvements. The major contributors in this research area are the United States of America and India. The study attempts to fully comprehend LM, its application, and trends to promote further research.


INTRODUCTION
In 1991, the book titled "The Machine that Changed the World" introduced the world to the term Lean Manufacturing (LM). [1] LM was derived from the Toyota Production System that gained popularity in the 20 th century as the least-cost production method by creating customer value and eliminating waste. After the revolutionary work of Womack et al., 1990, several authors (Oliver et al., 1994;Delbridge, 1998Delbridge, , 2003Pascal, 2002;Liker, 2004;Mann, 2005) have published books that help in understanding LM. The institutes which are the pioneers in LM are the Massachusetts Institute of Technology (SLOAN Management Institute), Cambridge-MIT (Centre for Competitiveness and Innovation), and Cardiff Business School.
The previous research publications have acknowledged the ability of LM in reducing inventories, improving quality, meeting the requirements of the customers at the right time, and reducing inconsistencies during the entire manufacturing. [2] LM gives the perspective for achieving higher productivity, market effectiveness, and internal efficiency, also exposes promising development of Lean study, to attain environmental and economic benefits. [3] Six Sigma is a strategy devoted to quality that can be linked and integrated into 'Lean Six Sigma'. [4] Lean processes are mainly concerned with reducing production time, eliminate waste in all the possible areas across the production lines. While Six Sigma focuses not only on defect elimination, also on project development, reduction of process variability, and costs. [5] Lean focuses on waste removal with six sigma, which reduces variation using statistical tools to achieve quality. [6] LM, over the years, has undergone many changes that have been conversed by plentiful researches. [7] Recent areas of interest related to LM have focused on organizational productivity, which is concerned with assigning employees for thinkers' roles by reducing human effort and promoting continuous improvement. Also, a comprehensive and holistic approach is involved in implementing LM in many organizations involving sustainability and green initiatives. A vast literature discusses the impact of the LM, both in studies of great scientific rigor and in books for practitioners, to consolidate these studies a bibliometric analysis is conducted.
Bibliometric analysis has been one of the most widely used statistical approaches for analyzing the published materials that have been gaining greater importance after the advancement of technology through computers and the internet. [8] The present study is conducted to understand the research trend on LM using bibliometric analysis with R software and Visualisation of Similarities (VOS) viewer. This study's structure is as follows: the second section contains the methodology, the third section shows analysis and quantitative results of R, followed by network representation in VOS viewer. Finally, the fourth section provides conclusions, discussion, and limitations.

METHODOLOGY
In this study, the Web of Science (WoS) citation database developed by Thomson Reuters Corporation was used for conducting the bibliometric analysis. WoS is the world's largest and most wide-ranging collection of information resources. [9] Bibliometrics is the routine use of citation analysis and a systematized approach to the processing data repository. [10] A topic search was performed using the keyword "lean manufacturing" to obtain published literature in the WoS database. Since the search was not restricted to any timespan, it extracted the documents since the inception of LM. The search was carried on December 14, 2020. It resulted in 1893 documents that were downloaded in. txt format for analysis. The files were imported to the open-source "Bibliometrics" package in RStudio software v1.3.1093 and executed to determine the highlighting features in the data extracted. Additionally, the VOS viewer software program was used for more interpretive network visualization of keywords, journals, publications, authors, and countries.

ANALYSIS AND RESULTS
The results obtained by the R bibliometric package highlight the characteristics of the WoS data. The summary of bibliometric information, topmost authors, institution, countries, and sources of contribution to the topic of LM. This critical information is vital for understanding the subject area's growth and prominent contributors to the development.

Summary of bibliometric information
The descriptive summary of bibliometric data is represented in Table 1. The total number of keywords used in the articles is 4454, 208 are single-authored documents, 4400 are multipleauthored documents. The average citation per document is 21.72, average citation per year per document is 3.115. The co-authors per document are 3.07, and the collaboration index is 2.7.

Publication growth pattern
As mentioned earlier, the topic of LM has gained much importance, and the number of articles published has been increasing. This is evident in the growth pattern Figure 1, which depicts the plot of peer-reviewed publications in the studies on LM. The most noticeable difference that can be inferred from the graph is that the trend is not exponential. The number of studies has varied alternatively. The Annual Percentage Growth Rate (APGR) of the articles published was found to be 11 %.
As the present study is focused primarily between 1970-2020, the few initial years had fewer articles published ranging between 1-15 between 1970 and 1997. Gradually, the number of articles published started increasing as the years progressed with various new areas related to LM. Due to advancements in technology and economic impact, the term productivity gained greater importance, which led to an increase in studies related to lean as a tool to increase profitability with the integration of other processes and quality improvement strategies.
An appropriate explanation for such a trend is Price's law, which explains that any research topic goes through four phases. At first, there is a phase called precursor, where fewer scientists begin to focus on the novel field and publish articles, followed by an exponential growth phase where the study area begins to grab the attention of various authors to conduct a study in different ways, later following the consolidation period during which the studies conducted becomes either iterative or stagnant, then slowly that particular area of research decreases. [11] Applying this theory to the LM, the  studies conducted have already reached the maturity point but still had new potentials to collaborate with topics like supply chain, six sigma, and performance and might gain importance.

Topmost contributing journals on LM
The top 10 journals which are frequently referred to by the researchers have been represented in Table 2. Key journals in the field are the International Journal of Productions Research and Production Planning and Control, with 164 and 107 publications on the topic.

Topmost contributing authors
Authors with the most articles published on the topic of LM are ranked in Table 3.
[Anonymous]A and Vinod S are the most productive authors with 26 publications each, followed by Tortorella Gl 21 publications.

Topmost countries contributing to LM
To continue characterizing the contributions geographically, researchers publish the most influential and productive countries.
The ranking in Table 4 is given mainly based on the total number of articles published, frequency of appearances in the journals, Single Country Publication (SCP), Multiple Country Publication (MCP), and MCP ratio. According to the findings, there is a wide range of countries publishing on the subject of LM. Countries' scientific output depends, among others, on economic and political factors. [12,13] For instance, the USA has the largest R&D investment of $134.1 billion. [14] Therefore, it is not surprising that its forefronts in the ranking of the most productive countries with 437 articles, followed by the UK with 159 papers. India in the third position having 144 articles.

Bibliometric analysis using VOS viewer
The study presents a bibliographic analysis using the VOS viewer software application. [15,16] The bibliometric techniques of co-citation analysis [17] and bibliographic coupling [18] are Conducted using this tool. The bibliometric metrics used to interpret the data are important to identify for bibliometric analysis. [19] The citations count from keywords, journals, sources, and countries is taken into account in the present study to reflect productivity, popularity, and impact.   Keyword co-occurrence The keyword co-occurrence is a bibliographical analysis applied to detect significant areas and classify those frequently co-occurring terms into theme-based clusters. [20] Bibliometric data reveals that this analysis included 6294 keywords. Results of keyword occurrence were carried out by co-citation analysis in VOS viewer for a minimum threshold of 30 keywords and the 300 representative co-occurrence links, as shown in Figure 2.
The size of each node is commensurate with the frequency of the keyword. The proximate nodes indicate those have simultaneously occurred in multiple articles, and the thickness of the links that connect them is proportional to the strength of the link. [21] Keywords such as performance, implementation, management, impact, and framework have high incidences, indicating a greater research focus, and are strongly associated with the other keywords. Since the keyword lean manufacturing was included in the search string hence its high occurrence is predicted.
Four clusters were identified, the largest red cluster with 21 keywords including implementation, framework, lean six-sigma is closely linked to keywords like performance and impact. The second-largest cluster was the green cluster with 20 keywords mainly containing management, productivity, supply chain, and performance are linked with critical success factors, continuous improvement. The blue cluster contained 13 keywords, such as systems, barriers, and sustainability. Other keywords like JIT, strategy, and operational performance are in the yellow cluster, indicating their link with LM. Table 5 shows the quantitative details relating to the top 10 highly-occurred keywords ranked based on their strength.

Prominent journals
Co-citation refers to the relation between two items that are both cited in the same document. [16] The co-citations study is extended to consider journals with a more significant impact on LM output growth. The co-citation study outcome for cited sources with fractional counting from VOSviewer, which has been at least 100 times cited, is 95 journals representing 100 co-citation links as Figure 3.
The gap between the two nodes indicates relative strength and the topic's similarity in the journals. [20]  The study related to the citation count and total link relation strengths in fractional counting can be found in Table 6.

Prominent publications
A related approach to prominent journals, co-citation analysis of referenced sources is adopted to obtain the most prominent publications. 49 publications meet the threshold of 50 citations out of 54594 cited references. The VOS viewer's results for the co-citation analysis are shown in Figure 4 with the most 200 representative citation links.
The analysis has resulted in four clusters, and the cluster size is small, indicating an increased accuracy. [22] A majority of the cited references are in a red and green cluster with 29 items and 19 items, respectively. The red cluster majorly focused on the onset of lean and lean thinking, which began in the The proximity of the clusters indicates a strong relationship as they are frequently cited. [23] Data obtained from the cluster analysis is available in Table 7,  ranked based on total link strength, indicating higher cocitations between researchers.

Prominent authors
The analytic unit is selected as "cited authors" in co-citation analysis in VOS viewers for establishing the influential authors in LM. Figure 5 Table 8 describes the data relevant to this network. With the enormous total link strength, Shah R indicates that other researchers have extensively co-cited his work.

Prominent institutions
The citation analysis with Organisations in VOS viewers is utilized for establishing the prominent institutions in LM. Figure 6 indicates the five primary clusters can be distinguished according to the most cited institutions by setting 8 minimum number of documents of an organization threshold resulting in 48 authors with the most 100 representative citation links.
The node size denotes the measure of publications, and the line between the two nodes demonstrates the academic link between the two institutions. The shorter the curve, the  Table 9 describes the data relevant to this network based on the total link strength. The Ohio State University has the highest citation of 2170.

Prominent countries
The most influential countries contributing to the development of LM are found by using a bibliographic coupling. A connection between two items that both cite the same document is a bibliographic coupling link. [24] The bibliographic analysis results for at least 30 citations per country, and the 100 stronger the link. It is inferred from the results that the red cluster has the largest items of 13 items of institutions such as   This pattern is expected as its already shown that the authors and publications from these countries are closely linked. Table 10 shows the ranking of the top 10 countries based on

DISCUSSION
LM is a vast subject that includes implementation and integration with various management techniques to obtain optimum results. A study has been published by Oliveira et al. 2018 about the trends and scientific development of Lean Manufacturing implementation. [24] It covers only the articles published between 2007 and 2018 from both Scopus and WoS databases. However, the present research has analyzed LM from its inception to develop a comprehensive overview of the progress of LM.
Also, a similar bibliometric analysis was carried by Sordon (2019) on the topic of Lean Six Sigma in manufacturing processes was conducted with 508 articles identified in the Scopus and WoS database between 2002 and 2017. [25] The survey analysis identified the most cited articles, keywords, productive journals, productivity, and collaboration. The study by Ciano (2019) contributes to the literature of lean specifically derived from research published in the International Journal of Production Research (IJPR) journal of Scopus database using VOS viewer software. The results obtained were confirmed by burst detection to compare the keyword occurrences of IJPR to other journals. [26] The previous works have not entirely focused on LM to give a complete understanding of the topic, its growth and integration with various topics, and its future implications. The research results have revealed that the highest publications and citations are given higher weightage, not the quality of high-impact work published in high-profile journals. In the future researchers must consider this point to promote the understanding of high-quality journals.

CONCLUSION
The new developments in the global economy have demanded a paradigm change from profit maximization to customer satisfaction in the manufacturing sector. One such processoriented approach for achieving sustainability in competitive markets is LM techniques. Our research provides a precedent that creates an opportunity to propose a repository of information to encourage LM-related studies.
The bibliometric analysis was performed on the data using the open-source R code and visualized using VOS viewer.
Mostly increasing growth pattern of the LM subject is found based on previous publications. The International Journal of Production Research has been described as the most significant source chosen by researchers to publish their work. Researchers [Anonymous] A and Vinod S have been actively contributed to the field. Based on the total number of papers written by countries, the USA ranks topmost, followed by the United Kingdom and India.
Network visualizations from VOS viewers show that LM has been used to integrate with other domains such as Industry 4.0, Six sigma, and adopting LM practices in an environmental, sustainable, and organizational performance perspective in developing nations. [27][28][29] LM practices are currently closely linked to green initiatives and sustainability performance. [30][31][32][33] Current research would provide insight into the pattern and be relevant for future LM researchers to identify gaps and explore new areas of interest.

LIMITATIONS
The study has limited its search to only articles indexed on the WoS database. WoS has higher research toward Natural Sciences and Engineering and Biomedical Research, [34] demonstrating many researchers might have used other databases to publish their work, such as Scopus and Google Scholar. This limitation also opens a research gap to compare our results with other databases.