A scientometrics survey on project scheduling

Article history: Received: October 28 2018 Received in revised format: November 25 2018 Accepted: January 23 2019 Available online: January 23 2019 In project management, a schedule is considered as a list a project's milestones, activities, and deliverables, normally with some start and finish time schedule, which are estimated by some information incorporated in the project schedule including resource allocation, budget, task duration, and linkages of dependencies and scheduled events. This paper presents a comprehensive review of the studies associated with project scheduling. The study uses Scopus database as a primary search engine and covers 3370 records over the period 1963-2019. The records are statistically analyzed and categorized in terms of different criteria. Based on the survey, "decision support systems" is the keyword which has carried the highest densities followed by heuristics methods. Among the most cited articles, papers published by researchers in Germany have received the highest citations (9084), followed by United States (7058) and Belgium with 4853 citations.


Introduction
In project management, a schedule is considered as list a project's milestones, activities, and deliverables, normally with some start and finish time schedule, which are estimated by some information incorporated in the project schedule including resource allocation, budget, task duration, and linkages of dependencies and scheduled events.Any schedule is normally implemented in the project planning and project portfolio management as parts of project management.Elements on a schedule are associated with different issues such as the work breakdown structure (WBS) terminal elements, the statement of work, etc. Project scheduling has been used in the literature for years (Krauss, 1963;Moshman et al., 1963;Betzig, 1964).The object of project scheduling and management is to produce a complete project by considering the client's desires.In several cases, the primary object of project management is to change the client's brief to address the client's objectives.When the client's targets become transparent, they ought to influence all decisions made by other parties involved in the project such as project managers, designers, contractors, etc.This paper presents an overview on studies associated with project scheduling.The study uses Scopus database as a primary search engine and analyzes the data over the period 1963-2019.

The most common keywords
Table 1 demonstrates some of the mostly cited references associated with project scheduling.As we can observe from the results of Table 1, Scheduling, Project scheduling and Project management are three well recognized keywords used in the literature.Fig. 1 shows the most important words used over times.

Table 1
The most popular keywords used in studies associated with project scheduling

Contributions of countries
Our survey demonstrates that European countries have maintained the most contribution in the field of project scheduling.Table 2 shows details of our survey.2, researchers from Germany have published 9084 papers followed by United States with 7058 papers and Belgium with 4853 papers.In terms of the average citation, papers published by researchers in Germany and Belgium have maintained the highest citations.Fig. 3 shows the results of the collaborations among various countries.

Fig. 3. Country collaboration map
As we can observe from the results of Fig. 3, there were strong collaboration from the researchers in United States from one side and other countries.

Highly cited papers
Table 3 shows the summary of the most cited articles.As we can observe from the results of Table 3, the study by Brucker et al. (1999) has received the highest citations.In their study, they provided a classification scheme, i.e. an explanation for the resource environment, the activity characteristics, and the objective function, respectively, which is consistent with machine scheduling and helps classify the most important models.They also proposed a unifying notation to review some of the recent developments such as exact and heuristic algorithms for the single-mode and the multi-mode case, for the time-cost tradeoff problem, etc.The second highly cited work is associated with Kolisch and Drexl (1997) where they provided a local search for non-preemptive multi-mode resource-constrained project scheduling.They proposed a general class of non-preemptive resourceconstrained project scheduling problems where activity durations were discrete functions of committed renewable and nonrenewable resources.They obtained a 0-1 problem formulation and explained the model by applying applications within production and operations management.In addition, they proved that the feasibility problem which is NP-complete and could hardly deal with some shortcomings.Thus, they proposed a new local search method that initially attempt to locate a feasible solution and then executed a single-neighborhood search on the set of feasible mode assignments.They also performed a computational study on two benchmark sets where the experiment included a comparison of the procedure with other heuristics.The third highly cited work belongs to Herroelen and Leus (2005) where they investigated a project scheduling under uncertainty.The other highly cited paper was accomplished by Merkle et al. (2002) which was a metaheuristics method named ant colony optimization for resource-constrained project scheduling.Hartmann (2010) in his remarkable work provided a competitive genetic algorithm for resource-constrained project scheduling.This work is one of the well-known non-review paper which has received a high citation and the average citation per year was also the highest for this item.A close look at the highly cited works listed in Table 3 reveals that many of them were associated with meta-heuristics methods ( (Boctor, 1990(Boctor, , 1993;;Boctor, 1996;Bouleimen & Lecocq, 2003;Debels et al., 2006;Debels & Vanhoucke, 2007;Gonçalves et al., 2008;Hartmann, 1998;Hartmann, 2001;Hartmann, 2002;Józefowska et al., 2001).This can be also verified in Fig. 4 where genetic algorithm has been used significantly.

Conclusion
This study has tried to provide a comprehensive review of the studies published in the literature associated with project scheduling.The study has indicated that this field has been popular mostly among researchers in United States, China, Germany and Iran.The study has also indicated that while researchers from Germany published a relatively high number of papers, they were also suc-cessful to publish highly cited papers.Many project scheduling studies have dealt with combinatorial optimization techniques and our survey has concluded that meta-heuristics methods have been popular among researchers to locate the near-optimal solutions.We hope this study could guide other researchers find important research gaps.

Fig. 1 .
Fig. 1.The summary of the most popular keywords used in project scheduling According to Table2, researchers from Germany have published 9084 papers followed by United States with 7058 papers and Belgium with 4853 papers.In terms of the average citation, papers published by researchers in Germany and Belgium have maintained the highest citations.Fig.3shows the results of the collaborations among various countries.

Fig. 4 .
Fig. 4. The frequency of the keywords used in different project scheduling studies5.Contribution of the countriesOne of the interesting areas of the interest is to learn more about the contribution of different countries in project scheduling.As we can observe from the results of Fig.5, researchers from China (537 papers), United States (487 papers), Iran (257 papers) and Germany (215) have contributed the most on project scheduling.

Fig. 5 .
Fig. 5.The frequency of the keywords used in different project scheduling studies

Table 2
The summary of the contributions of different countries

Table 3
The summary of the most cited articles