Chapter 25: A Comparison of Drum-Buffer-Rope (DBR) and Critical Chain (CC) Buffering Techniques

Van Gray, Ph.D.Baylor University
Joe Felan, Ph.D.Baylor University
Elisabeth Umble, Baylor University
Michael Umble, Ph.D.Baylor University

Introduction

Project-based management has long been the business style associated with the construction industry, United States (US) defense contractors, large consulting firms, and even Hollywood. Today, project management is spreading to all sectors of work. Accordingly, the membership in the Project Management Institute (PMI R), a professional organization for the project management specialist, quadrupled between 1993 and 1997. PMI membership is growing by some 1,200 members per month and is expected to break the 100,000-member mark by 2002. Clearly, the trend indicates that there will be an increase in the importance and role that projects and their successful management contribute to the strategic direction of the firm. David Cleland, an established project management scholar, has declared that this is the "Age of Project Management" (Cleland 1990).

At the same time that project management methods have become common practice, the pressures associated with global competition, increased customer focus, shortened manufacturing lead-times, and corporate downsizing have created many new challenges for traditional project management. Among these challenges are the issues of sharing resources and prioritizing those resources across a portfolio of concurrently implemented projects. Resolving the conflicts that arise during this resource sharing and prioritization is what really converts the project network into a project schedule.

Historically, the project management literature has made very little connection between the management of manufacturing activities necessary to create a variety of finished goods and the management of activities required to successfully complete a project. Despite the fact that every organization undertakes projects, manufacturing-based organizations and project-based organizations are generally perceived as being dramatically different. In fact, project managers have traditionally viewed the repetitive tasks of manufacturing a tangible item and the typically one-time, non-repetitive tasks associated with a project as being so different that a completely different set of tools is required to plan, schedule, and control the activities. Manufacturing activities are best coordinated and controlled using one of the many best practices of the day in combination with some variation of a material requirements planning (MRP) system, manufacturing resource planning (MRP II) system, or enterprise resource planning (ERP) system. But conventional wisdom suggests that project activities are best coordinated and controlled using variations of the critical path method (CPM) or program review and evaluation technique (PERT).

However, upon closer examination, it becomes obvious that both manufacturing-based and project-based organizations share a common set of objectives. Both must meet the expectations of their clients with on-time delivery, while adhering to budget limitations necessary to achieve levels of profitability acceptable to their stakeholders.

It is not unreasonable to believe that a general body of knowledge already exists that could be utilized to achieve the common objectives of both organizational types. The authors of this chapter suggest that such knowledge does, in fact, exist. The body of knowledge is broadly referred to as Theory of Constraints (TOC) or Constraint Management (CM). This approach, developed by Dr. Eli Goldratt, was first described in his now classic book, The Goal (Goldratt 1984, 1986, 1992). In this business novel, Dr. Goldratt provides the basic decision framework to solve numerous business problems. Those who are familiar with the book will remember the basic framework as the five focusing steps. We now describe in detail how these five steps provide the framework for coordinating and controlling activities in both manufacturing and project environments.