Critical chain project management

Abstract

This literature review assesses the current stage of research investigating Critical Chain Project Management (CCPM) and examines the role that academic researchers played in response to the introduction of CCPM concepts and techniques. First, this article identifies the research manuscripts published in peer-reviewed academic journals by year, subsequent to the release of Critical Chain by Eliyahu Goldratt in 1997. Next, this article delineates the roles played by project management researchers, which includes introducers, evaluators, developers, cross-pollinators, and implementers. This article describes and presents the main contributions associated with each of these roles. This study also examines the leadership role played by researchers by highlighting the future research efforts recommended in the articles reviewed. This article concludes that critical chain project management remains under investigation and the authors recommend further study to determine when CCPM should be implemented, how CCPM should be implemented, which obstacles should be avoided, and what benefits are likely to be realized.

Keywords: critical chain; project scheduling; buffer management

Introduction

In 1997, the business novel, Critical Chain, was published by Eli Goldratt. In short order, the ideas expressed by Dr. Goldratt precipitated a substantial interest among project managers (Cabanis-Brewin, 1999). Proponents claimed that Critical Chain presented a totally new, revolutionary way of thinking with the potential to achieve superior performance in terms of delivery times and schedule commitments (Raz et al., 2003). The enthusiasm surrounding the publication of Critical Chain led Dr. Goldratt to suggest that these ideas were perhaps among the biggest breakthroughs in project management since PERT and CPM (Cabanis-Brewin, 1999). The potential benefits to be realized through application of the critical chain methods served as a catalyst to capturing the interest of project managers the world over.

In the wake of this enthusiasm, a significant responsibility was placed squarely on the shoulders of the project management research community. As interest in critical chain grew among practitioners, it became incumbent on the research community to investigate the concepts presented by Dr. Goldratt. The thirst to apply the concepts in contemporary organizations beckoned the services of impartial researchers to first evaluate the concepts, to calibrate the potential benefits, and to validate the implementation of the methods in a realistic environment. Important questions required answers. Are the critical chain methods sound? How do critical chain methods compare with others frequently used? What are the strengths and weaknesses of the critical chain approach? What challenges face implementers? Are the methods better suited for certain organizations over others or certain projects over others? Nearly 15 years have passed since Critical Chain was published. This period of time has provided ample time to make significant progress in fulfilling the research responsibilities mandated by the needs in the project management community. Hence, it is now time to assess our progress. Accordingly, the purpose of this article is to investigate the manner in which the project management research community has fulfilled its responsibilities to evaluate Critical Chain Project Management (CCPM).

Objectives

This study pursues four primary research objectives to investigate the research conducted with regard to critical chain project management. Specifically, this study will:

1. Identify the critical chain research manuscripts published between 1997 and 2009.

2. Identify and examine the roles played by researchers who conducted critical chain research.

3. Examine the relationships between author affiliation and the various roles played.

4. Identify future research and further studies recommended in the literature.

Method

To be sure, the academic community has relied upon a variety of research outlets to introduce, evaluate, and report research concerning critical chain project management. These outlets have included theses and dissertations (e.g., Ingram & Scherer, 1992; Pittman, 1992), proceedings from conferences and symposia (e.g., Simpson & Lynch, 1999), and articles in practice-focused periodicals such as PM Network^® (e.g., Pinto, 1999). The early publications also included book reviews (e.g., Elton & Roe, 1998; McKay & Morton, 1998), and published books (e.g., Newbold, 1998). The early efforts (e.g., Ingram & Scherer, 1992) helped to blaze the trail in the development of the critical chain concepts by applying the theory of constraints to project scheduling. Additionally, some of the important initial evaluations of the critical chain (Pinto, 1999) were published in the practice-focused periodicals capitalizing on the shorter lead times to publication and broader distribution of the periodicals. However, for the purpose of this research, we have elected to confine our sample to research manuscripts published in peer-reviewed academic journals. The peer-review process helps to ensure the rigor and quality of the articles we have included in the sample. Additionally, our reliance on peer-reviewed journals helps to ensure that interested researchers will also be able to access the articles reviewed. Finally, we are able to conduct a more thorough search by limiting our sample to peer-reviewed journals.

The first step was to identify and collect all of the articles published between 1997 and 2009 in peer-reviewed journals that addressed critical chain project management. We relied heavily on electronic searches, using the following search engines: EBSCO, JSTOR, and Google Scholar. To perform this search we screened for keywords to include “critical chain” and “CCPM.” This initial review generated 61 candidate articles. We then reviewed the reference lists in each of these articles to identify any additional journal articles that we may have missed relying on the search engines identifying an additional nine candidate articles. The next step was to eliminate articles that were not peer-reviewed research manuscripts. During this activity, we screened each article for evidence that a review was performed as well as evidence of scholarship. In some cases, when we encountered publications with which we were not familiar, we investigated the source further. We eliminated book reviews, short essays, and proceedings articles. We also eliminated any articles that were not written in English. The result of this search and screen process yielded a total of 38 peer-reviewed journal articles.

Once the collection process was complete, we reviewed each of the articles and coded them to support our analysis effort. The coding process logged the year of publication, assigned each article to a specific role category, identified the author affiliations, and captured any recommendations for future research. To achieve the research objectives for this effort, we tallied the totals of observations and presented these descriptive statistics in the figures and tables to follow.

Results

Articles Published

The first objective of this study was to identify the critical chain research published between 1997 and 2009. These results are presented in Figure 1.

 

Figure 1: Total articles published by year.

An interesting finding emerges when we consider the annual rate at which the articles were published during the period of study. It appears that the number of articles increased during the first half of this period as interest in critical chain project management grew. Perhaps somewhat more surprising is that it appears that the publication rate for critical chain articles declines during the second half of the period subsequent to the peak year of 2005. What are the factors that explain this decline? Has the academic community answered all of the important questions? Have the concepts and methods been proved to lack merit? Have the purported benefits failed to materialize in practice?

Roles Played

These questions inspire our second research objective—what roles have researchers played with respect to critical chain project management? As we reviewed the literature, we discerned the articles could best be characterized by assigning them to one of the following five categories:

1. Introducers—emphasis placed on presenting and explaining the concepts.

2. Evaluators—emphasis placed on examining and critiquing the concepts.

3. Developers—emphasis placed on development of the concepts and techniques.

4. Cross-Pollinators—emphasis placed on adapting the concepts to new domains.

5. Implementers—emphasis placed on detailed analysis of implementation of the concepts in a real organization.

Figure 2 provides a summary of the peer-reviewed articles published during the research period based on these role categories.

 

Figure 2: Roles played by researchers.

The Introducers. A majority of the early articles were dedicated to introducing Critical Chain Project Management (CCPM). These articles typically presented the essential concepts associated with CCPM and relied heavily on Goldratt's book, Critical Chain (1997); we can summarize these concepts as follows. First, Goldratt maintains that project managers insert safety into their estimates of activity durations. He asserts that project managers base their estimates on pessimistic experience. Then, each level of management adds safety and, finally, estimators protect their schedules from a global cut. The consequence of these practices is that schedules will be bloated with safety. Once safety is introduced into the schedule, Goldratt explains that several mechanisms act concurrently to waste the safety. Specifically, he cites student syndrome or the tendency to wait until the last minute, along with multi-tasking, delays caused by activity path merging, and the fact that dependencies between steps will cause delays to accumulate and advances to be wasted. Goldratt then proposes an alternative approach that promises improved schedule performance. Essentially, he advocates rendering more aggressive estimates of activity durations and then pooling the safety harvested from each activity on the critical chain and placing it in a project buffer that will protect the project due date. In a similar fashion, safety that is harvested from non-critical chain activities is pooled in feeding buffers that protect the critical chain. Goldratt defines the critical chain to be the longest chain of dependent steps, recognizing that dependencies can be the results of precedence relationships or reliance on a common resource. In recognition of the importance of critical resources, Goldratt also advocates the use of a resource buffer, which essentially serves as advance notice to resources responsible for accomplishing critical chain activities. Finally, control of the project is accomplished by management of the buffers. Consumption of the safety in the buffers in excess of established thresholds triggers management action.

In addition to presenting these concepts, some of the introducer articles (Leach, 1999; Umble & Umble, 2000; Blackstone et al., 2009) illustrated the essential concepts with notional data. As would be expected, most of the articles that introduced the concepts were among the first articles published (see Figure 2).

The Evaluators. One of the most important roles to be performed by the research community is to evaluate emergent concepts and to place the advances in context of the work that preceded the advance. Perhaps the article most dedicated to this purpose was published by Trietsch (2005a). Trietsch makes the case that many of the key concepts of CCPM, such as the use of resource constraints rather than resource leveling, the elimination of due dates to manage critical activities, the use of project buffers and feeding buffers, and the avoidance of multi-tasking were previously addressed in the academic literature. According to Trietsch, the principal contribution that Goldratt made was to package several items that go well together, and in doing so he created a more holistic approach to project scheduling than was previously the typical case.

Herroelen and Leus (2001) contributed a very rigorous evaluation of critical chain/buffer management (CCPM/BM). Their work focused on the procedures for identifying the critical chain in the baseline schedule, buffering the baseline schedule, and using the buffers as a proactive mechanism. They also addressed the issue of stability of the baseline schedule and provided strong support for the importance of rescheduling. Their observations are based on their experiences as well as a rigorous full factorial experiment described in Herroelen and Leus (2001). These authors sought to provide a balanced view of CCPM/BM, citing both strengths and weaknesses of the approach. Because we are concentrating on the evaluative contribution of this research, we share the following concerns and issues identified by Herroelen and Leus (2001) and Herroelen et al. (2002).

1. CCPM promotes the minimization of project duration as the primary objective and performance measure and fails to address other practically relevant measures to include net present value of projects, minimizing resource availability costs, or maximizing project quality.

2. CCPM focuses on the assignment of single unit renewable resources, such as human beings to accomplish project tasks. Activities requiring several units of the same resource type or activities requiring non-renewable resources such as money, energy, and space do not receive significant attention.

3. It is very likely that there can be more than a single critical chain. Although Goldratt assumes that identification of the critical chain is easy, in fact, it is an important strategic decision, which is definitely not easy to make.

4. A common CCPM approach is to set the size of the project buffer to 50% of the project duration. In many cases, this approach to sizing the project buffer could provide an unnecessarily large amount of protection against variations and lead to uncompetitive proposals and loss of business opportunities.

5. CCPM adds the feeding buffers after the project buffer has been added. It may be more accurate to calculate the size of the project buffer after the feeding buffers have been inserted.

6. CCPM employs the roadrunner mentality in which an activity begins as soon as the predecessor activities are completed. This approach implies that two different baseline schedules must be maintained. The first is a late start–based schedule with the feeding buffers inserted. The second is an early start–based schedule that does not take the buffers into account. With both schedules, the critical chain activities are kept in series. This dual schedule approach may be difficult to communicate to the workers.

7. Buffer consumption often implies resource contention and the need to resolve resource conflicts somewhere in the schedule. These repair activities may delay critical chain activities.

8. CCPM resists changes to the baseline schedule during project execution, except for major disruptions. However, project execution often involves activity delays, the insertion of new activities, late vendor deliveries and so forth. Consequently, the CCPM approach may retain focus on a critical chain, which will no longer determine the project duration and forego the opportunity to speed up the project in response to changes that occur during execution.

In 2003, Raz et al. contributed a highly evaluative article to the debate of the merits and pitfalls of critical chain project management. Their concerns addressed a range of items, including behavioral and organizational issues. We provide the following summary of their major concerns:

1. CCPM proponents fail to provide scientific evidence to support two of their most important underlying assumptions; namely, that all task owners overestimate task durations by a certain safety factor and then allow the time required to complete the task to expand to the time allotted.

2. CCPM proponents have yet to adequately address the behavioral aspects associated with removing a precise amount of safety from task owners to pool it in the project buffer. How do task owners respond to this change in approach?

3. CCPM proponents have yet to articulate a clear scientific basis for sizing the buffers.

4. During execution, CCPM relies on task owners to report the duration remaining. Why would the estimate of duration remaining be any more accurate (and free of safety margin) than the original task estimates that CCPM constrains?

5. In a multi-project environment, CCPM prioritizes resource allocation based on the amount of buffer consumed, to the exclusion of other important considerations, such as the nature of the penalties incurred for missing project deadlines or the strategic impact of the project.

6. CCPM prohibits the assignment of a resource to more than one concurrent task; however, there is research that suggests that assignment of resources to 2 or 3 concurrent projects may improve firm performance.

7. CCPM relies on resources beginning the next task as soon as possible. This policy requires a great deal of unscheduled communication, which may prove very problematic within the organization and possibly infeasible if some of the resources are outside contractors unable to drop their on-going tasks to invest their full attention on this next task.

8. CCPM assumes there is a single critical resource (the drum) that determines the critical sequence. This assumption is based on a steady state view of the work mix, which may be applicable in manufacturing and operations environments but may not be applicable to most project environments where there is no steady state.

9. CCPM focuses on a single performance objective—the schedule, to the exclusion of numerous important objectives such as profitability and business effectiveness.

10. CCPM addresses the inherent uncertainty in project management through the use of buffers instead of adopting risk management techniques that are designed to identify and mitigate the underlying sources of uncertainty.

11. CCPM implementation requires the use of a software tool, and the range of software tool options is limited and relatively expensive.

12. CCPM implementation imposes pervasive changes to how projects are managed, including buffer calculations, personnel assignments, and progress reporting. These changes mandate significant education and training at various levels throughout the organization

Another important role to be played by evaluators was to compare critical chain project management with the other contemporary methods. This challenge was embraced by two separate teams of researchers who relied on simulation modeling to compare CCPM with the more traditional critical path method. These researchers concluded that critical chain, when properly implemented, can provide a significant time-saving advantage over the critical path method (Budd & Cooper, 2005), and that many of the key ideas of critical chain project management can be easily adapted by practicing project managers (Lechler et al., 2005). Finally, another noteworthy study relied extensively on simulation modeling to compare control mechanisms in a multi-project environment (Cohen et al., 2004). These researchers compared buffer management as implemented in CCPM with other contemporary control mechanisms such as Highest Priority in Queue to a Minimum Slack Activity (MinSLK) and Constant Number of Projects in Process (ConPIP), among others. They concluded that in a multi-project environment typical of organizations in the aircraft industry, buffer management may not be enough to meet the planned schedule. Some of the alternative mechanisms yielded similar and sometimes better performance.

The Developers. Although the introducers played an important role in sharing and illustrating the concepts of critical chain project management, many of the early articles were largely just that…conceptual. This is also true of Goldratt's Critical Chain. A challenge that remained was to further develop the concepts and techniques to a point at which they could be readily implemented in contemporary project performing organizations. This challenge inspired the efforts of the developers.

One of the principal areas addressed by researchers whose contributions developed critical chain concepts was the topic of sizing the buffers. The first of these articles appeared in 1999 (Hoel & Taylor) who relied on Monte Carlo simulation to demonstrate the relationship between the size of the project buffer and the probability of completing the project on time. These authors also concluded that project schedulers need not create feeding buffers, as they are provided naturally by the free slack in a project network. This conclusion represents a departure from the approach advocated by Goldratt, which first determines the critical chain considering the specific resources associated with activities.

Trietsch (2005; 2006) adopted a theoretically more rigorous approach to investigate the determination of optimal buffer sizes. His work relied on mathematical modeling to address both project buffers (2005) and feeding buffers (2006). Tukel et al. (2006) also contributed methods to determine feeding buffer sizes. These authors relied on a simulation study to test two buffer sizing methods that integrate project characteristics into the formulation. Their test results indicated that consideration of resource tightness and network complexity both generated smaller buffer sizes while providing sufficient protection against delays in project completion time.

Finally, Agarwal and Borchers (2009) shifted the focus from the sizing of buffers to the management of the buffers. They proposed a two-dimensional metric they call the Buffer Burn Index that considers both the percentage of buffer penetration and the percentage of job complete. The authors suggest this metric holds potential for managing project performance proactively in a multi-project environment. The authors demonstrated the application of this metric with a real case scenario.

The Cross-Pollinators. The role category of cross-pollinator was created for those articles in which the principal focus was to apply and integrate CCPM concepts into new dimensions of project management, to specific practice domains, and to emergent resource constrained project scheduling approaches. The principal attribute that distinguishes this category of research articles is that the emphasis was placed on accepting and applying the CCPM concepts rather than evaluating and critiquing them.

Several researchers dedicated their efforts to applying CCPM concepts and techniques to new dimensions of project management beyond project scheduling. These researchers applied Theory of Constraints (TOC) principles in general and CCPM concepts in particular to a wide variety of project management processes. For example, Leach (2003) applies the concepts of using buffers to account for the variability in cost estimates as well as schedule estimates. Steyn (2002) applies TOC principles to managing resources shared by a number of concurrent projects as well as to risk management. The process of project control, including earned value management, is adapted to integrate CCPM concepts in a model proposed by Hegazy and Petzold (2003), whereas Taylor and Rafai (2003) applied CCPM techniques to the strategic budgeting process. Finally, Yeo and Ning integrated the CCPM concepts into a framework for supply chain management (2002; 2006).

In addition to applying the CCPM concepts to project management processes, some researchers reported on the application of the concepts to various practice domains to include advertising (Budd & Cooper, 2004), construction planning (Winch & Kelsey, 2005; Yang, 2007), and maritime disaster rescue (Yan et al., 2009). In most cases, these researchers demonstrated the application of the concepts using hypothetical data sets.

The third set of cross-pollinators integrated CCPM concepts and techniques into emergent approaches to resource-constrained project scheduling. Rivera and Duran (2004) proposed an algorithm to determine critical sets and critical clouds, which serves as an unambiguous procedure to establish the critical sequence. Rabbani et al. (2007) developed a new heuristic for resource-constrained project scheduling in stochastic networks using CCPM concepts. Long and Ohsato (2008) developed a fuzzy critical chain method. To this end, they rely on fuzzy numbers to size the project buffer. Finally, a fourth article in this set applies critical chain scheduling techniques to Grey systems (Li & Feng, 2008). In all four of these articles, the authors relied on mathematical modeling to develop the method that incorporates critical chain concepts.

The Implementers. Despite the relatively large number of research articles to address critical chain project management during the period of this study, we identified only two articles that performed a reasonably thorough and structured investigation of the implementation of CCPM in a realistic environment. Both of these articles were published in recent years, a full decade after the release of Goldratt's Critical Chain (1997).

The first article (Srinivasan et al., 2007) was a detailed case study that investigated the implementation of critical chain concepts and methods into the C-5 maintenance, repair, and overhaul (MRO) process at Warner Robins Air Logistics Center in Georgia, USA. This logistics center provides MRO services for the United States Air Force. The implementation applied several key concepts associated with critical chain project management. Activity estimates were made aggressively, such that the total project time was set at 50% of the original estimate. One half of the time cut was added back as a project buffer. The team identified 120 paths that fed into the critical chain and inserted feeding buffers into each. The feeding buffers were also set at 50% of the duration of the feeding chain. Concerto software was used to identify the critical chain. The project team received training about CCPM concepts prior to the implementation. Schedule performance was managed by monitoring the status of the buffers. The primary benefits realized through this implementation included increased maintenance workload and the associated revenues achieved through scheduling efficiencies, as well as non-quantifiable benefits to include increased responsiveness and casualty avoidance during wartime.

The second case study (Bevilacqua et al., 2009) detailed the implementation of the Theory of Constraints to include critical chain concepts at an Integrated Gasification Combined Cycle (IGCC) plant at an oil refinery in Falconara Marittima, Italy. The project schedule included a project buffer, feeding buffers, and resource buffers. The project buffer and feeding buffers were sized using a calculation that based the buffer size on the aggregation of risk along the chain that feeds the buffer. The critical chain was determined by first creating a preliminary plan to identify the longest path in the network. Next, the team balanced the load, and assigned the tasks to resources, using a backward pathway and a priority index. They then determined the critical chain as typically defined; that is, the set of tasks that determines the overall duration of the project, taking into account both the dependencies and the resources. The project team used Project Scheduler 8 software to develop the schedule. The researchers reported that critical chain planning yielded considerably shorter task durations and increased the reliability of the completion dates. Also, they indicated the need for frequent scheduling changes was reduced. These results were based on comparisons with a similar, previous turnaround project conducted at the same plant the previous year, using CPM rather than critical chain scheduling.

Role of Affiliation

The third research objective examines the relationships between author affiliation and the roles played by the researchers. This objective is motivated by questions like the following: Do we see greater involvement among practitioners and consultants in articles that introduced the concepts? Do committed academics play the dominant role in completing the evaluative articles? Does the participation of a practitioner or consultant help to enable access to a real organization that has implemented CCPM?

Table 1: Role category and researcher affiliation.

The results are inconclusive, because the sample size in each category is relatively small. From a purely descriptive perspective, it is interesting to note that practitioners and consultants participated in supporting each category of research either independently or on a mixed team. Indeed, 9 of the 38 articles benefitted from the contributions of authors affiliated with a commercial entity. It also appears that the authors who completed evaluative articles reflect a higher proportion affiliated with academic institutions. Additionally, one of two articles that investigated the implementation of CCPM in a real organization relied on the participation of researchers affiliated with the organization investigated.

Leading Future Research

The fourth objective of this research was to examine the leadership role played by researchers. Collectively charged with the responsibility to apply rigorous scrutiny of the emergent concepts of CCPM, researchers can play an important role by recommending future research efforts. If this role is performed conscientiously, over time the community of researchers who pursue well-conceived research recommendations should be able to fulfill its responsibilities. Specifically, the articles we reviewed recommend that future research should:

1. Investigate and scientifically validate assumptions regarding human behavior. For example, behavior during planning could be examined using several methods to include surveys, interviews, and direct observations of individuals estimating activity durations and cost (Steyn, 2000).

2. Investigate the practicality of project execution. Case study research could provide evidence to validate the rigor of CCPM for project time management (Steyn, 2000).

3. Develop effective and efficient algorithms to create robust baseline schedules (Herroelen & Leus, 2001).

4. Develop intelligent scheduling/repair algorithms (Herroelen & Leus, 2001).

5. Develop mechanisms to warn project management of emerging problems during project execution that may compromise the project due date. These mechanisms should also enable dynamic evaluation of the criticality of project activities (Herroelen & Leus, 2001).

6. Develop more robust scheduling and control mechanisms for multi-project stochastic environments. This research could begin with “mini-system” analysis and then implement the findings in realistic environments (Cohen et al., 2004).

7. Compare organizations that have adopted CCPM to determine if they perform better than organizations that apply a conventional project management methodology (Raz et al., 2003)

8. Examine how to monitor buffers without tampering and based on statistical quality control principles (Trietsch, 2005).

9. Identify the necessary and sufficient conditions for CCPM to work and to test its robustness (Lechler et al., 2005).

10. Examine the effectiveness of the Buffer Burn Index metric using case studies in real organizations (Agarwal & Borchers, 2009).

11. Conduct case studies to investigate the application of TOC to single project scheduling, multiple project scheduling, cost management, and risk management (Steyn, 2002).

12. Investigate the application of TOC to the problem of project selection (Steyn, 2002).

13. Investigate the coupling of supply chain management and CCPM (Yeo & Ning, 2002).

14. Explore the effects of multi-tasking and implementing CCPM in multi-project environments (Budd & Cooper, 2004).

15. Identify behavioral barriers to CCPM implementation as well as client reactions and perceptions of the benefits realized through CCPM (Budd & Cooper, 2004).

16. Empirically test CCPM against PERT/CPM in a realistic setting with comparable conditions (Budd & Cooper, 2004).

17. Develop an efficient method for determining project buffer size in particular situations, so that the buffers will take into account factors that may vary by context (e.g., weather, labor skills, and equipment for construction projects). (Long & Ohsato 2008)

Discussion

The title of this article raises an important question: Is critical chain project management still under investigation or is the case closed? A striking observation that emerges from a review of the research articles published between 1997 and 2009 suggests that perhaps the investigation has lost its steam. Does this mean the questions are answered? Can we recommend that organizations implement CCPM without reservation or condition? Can we dismiss it outright? In this article, we have examined the research and the roles played by researchers to determine the current status of critical chain project management. We discovered that in the period shortly following the release of Critical Chain (Goldratt, 1997), the principal roles played by researchers were to introduce the concepts and to demonstrate the concepts with notional data. Indeed four out of five articles published in 1999 and 2000 fulfilled this important role. Once introduced, researchers assumed the responsibility to evaluate the concepts and techniques. As indicated in Figure 2, nearly 40% of the articles published between 2001 and 2005 focused on the evaluation of CCPM. The evaluators placed the concepts and techniques of CCPM in the context of the work that preceded the publication of Critical Chain (Goldratt, 1997). In several cases, the evaluators demonstrated that many of the concepts were not new (e.g., Trietsch, 2005a), or that the manner in which they were introduced would not yield optimal results (e.g., Herroelen & Leus, 2001). In some cases, the evaluators indicated that proponents had yet to fully specify how to best implement CCPM or to fully defend the underlying assumptions on which the concepts are derived (e.g., Raz et al., 2003). Moreover, the evaluators raised important questions regarding whether CCPM was universally applicable or best suited for specific types of projects and organizations (e.g., Herroelen & Leus, 2001; Raz et al., 2003). Despite these reservations and concerns, most evaluators acknowledged that Goldratt raised greater awareness and recognition that the interaction between activity durations, precedence relations, resource requirements, and resource availabilities has a major influence on project duration (e.g., Herroelen & Leus, 2001).

In many cases, the concerns identified by the evaluators were not so much a repudiation of CCPM as they were calls for further development and clarification. But who is responsible for this further development? Does this responsibility fall on the shoulders of the research community or solely on the proponents of the concepts? We examined the role the developers have played to date. To a very large extent, the developers have focused on techniques that can be used to size the buffers and manage their consumption. It is our sense that there remains a significant opportunity to further develop the concepts and techniques of CCPM. Finally, we reviewed the roles of the cross-pollinators and the implementers. The cross-pollinators have applied TOC concepts and CCPM methods in a variety of domains. An important contribution of these articles is that they signal the potential benefits to be realized from CCPM and serve to motivate future research. Of more interest to us is the research provided by the implementers. To date, only two articles reported in peer-reviewed journals provide a detailed account of the implementation of CCPM in a real organization. These two articles represent a critically important step forward, but with only two detailed case studies, we must conclude that this investigation should remain open.

Our fourth research objective was to examine the future research recommended by the articles we reviewed. If indeed this investigation should remain open, which questions should be answered? Which research objectives should be reviewed? The 17 research topics indentified in the results section provide several worthwhile ideas for future research. We will share some of our own ideas in the following section.

Conclusion

It is our principal conclusion that the case is certainly not closed! Although the level of research activity appears to be on the wane, the community of project management researchers has yet to completely fulfill our responsibility to provide objective scrutiny of the CCPM concepts and techniques and to accomplish the further development required so that practitioners will have a more complete understanding of when CCPM should be implemented, how CCPM should be implemented, which obstacles should be avoided, and what benefits are likely to be realized. We conclude this article with some specific recommendations for future research, which, if pursued, will advance our efforts to fulfill our responsibilities as researchers.

First, we must determine when CCPM represents a suitable approach for an organization. If it is not universally applicable, are there certain project management environments, types of projects, or types or organizations that are more suitable for the implementation of CCPM? Many of the articles that introduced the concepts have adopted the premise that a single renewable resource is assigned to each task in the network. This assumption drives the policy of identifying and managing the critical resource that determines the critical chain. There are numerous projects, especially in the systems development and product development domains that rely on the collaboration of teams of knowledge workers. These domains are also more likely to experience very high uncertainty and the unforeseen addition of new tasks (Herroelen & Leus, 2004). Raz et al. (2003) argue that projects that begin with design and analysis experience more complex network flows. These issues raise the question: Is CCPM suitable for use in these environments? The two case studies reported to date both describe large maintenance operations in large facilities. These environments are more akin to the maintenance and operations environments where the TOC has flourished. A major question to be answered by future research is whether CCPM concepts and techniques are suitable for certain types of projects and organizations, and if so which ones?

Second, if we have determined that CCPM is suitable for a particular organization, then how should it be implemented? The work of several of the evaluators identifies research topics to improve the application of the scheduling CCPM scheduling practices. Future efforts should address how to best identify the critical chain, how to best size the buffers, when to insert the feeding buffers, and when and how to repair or reschedule the baseline schedule once project execution is underway.

Third, organizations that commit to implementing CCPM must be aware of the obstacles they are likely to encounter; and, where possible, which strategies might prove useful to avoid, mitigate, or overcome the obstacles. Case studies could prove to be an invaluable source of assistance in this regard. There are numerous implementation issues that are raised in the literature. What methods have been used to secure subcontractor, supplier, or third-party integration? How have they worked? What methods of communicating timely scheduling information have been used with external parties? What methods have external parties used to communicate their progress on critical activities? Similarly, how have organizations with a mandate to provide earned value reporting integrated buffer management? How do CCPM and the metrics involved support cost performance measurement and reporting? The nature of these questions and many more underscore the need for more case studies and a greater variety of case studies. Future case studies must approach the implementation objectively and characterize the context specific attributes of the organization that may impact the integration efforts. They should describe how the implementation was managed, as well as the obstacles encountered and addressed. They should describe the metrics used to assess the success of the implementation effort. And they should report these results through a peer-reviewed process that ensures that rigorous academic standards are maintained.

Finally, additional research is needed to validate the benefits to be realized through the implementation of CCPM. Many of the articles we reviewed pointed to success realized through implementation efforts at organizations, which include Harris Semiconductor, Lucent, an Israeli aircraft company, and the Lockheed Martin F-22 wing assembly operation, among others. Often, the metrics used to characterize the successes include makespan, throughput, work in process inventory, and so forth. These metrics again raise the question if CCPM is best suited for environments that are more akin to manufacturing, maintenance, and operations. Once again, there is a need for a wider variety of case studies that measure success in terms more typical of project management environments, such as schedule performance, cost performance, product quality, customer satisfaction, and business-related benefits. Some of the previous research has pointed to significant costs associated with implementation of CCPM. In particular, they have identified the cost of the requisite software, along with the cost of the substantial education and training efforts to prepare all participants to be involved with the implementation efforts. We recommend that future research should account for both the benefits to be realized as well as the costs to implement CCPM.

Clearly, the jury is still out. Researchers have contributed significantly to our understanding of the concepts of CCPM. Researchers have also shed greater light on the merits and shortcomings of CCPM. But it is too soon to close the case. There remain a considerable number of questions that warrant further investigation and it is time to reinvigorate the search.

©2012 Project Management Institute