Theory of Constraints Handbook - James Cox Iii [28]
Cause: The project network is not developed to include all obstacles that must be overcome before an activity might begin.
This is addressed by Guideline VII.
Micro Issues
The micro issues relate to errors or shortcomings in using the project tools. We will use simple numerical examples of each problem. By careful study of these errors and their causes, a systems approach to project planning, scheduling, and control can be developed and tested to ensure it addresses each of these problems.
Gedanken exercises, or thought experiments, have traditionally been used in the sciences rather than in business. The method uses logic and simple mathematics to construct an illustrative example to validate a hypothesis. While the method has usually been applied to scientific research areas such as quantum mechanics or astral physics where time, space, or both separate the subjects of scrutiny from the researchers, gedanken exercises also have the advantage of holding all other variables constant in order that the effects of the variable being examined are isolated. This simplification allows the researcher to gain knowledge and understanding by examining fragments of the system one piece at a time rather than losing the effects of an individual variable in the noise of many interacting variables. With a full understanding of the behavior of each variable acting in isolation, the researcher might be able to construct a logically sound theory about the system.
The use of gedankens in this research is based upon the realization that there are many factors that contribute to the project completion delays found in project planning, scheduling, and control. Here, the use of gedankens allows for the examination of each factor in isolation so that the factor can be studied to determine its effects on project completion.
Single Project Gedankens
Problem 1: Variability and Convergence Points The first of the eight weaknesses attributable to the assumptions of PERT/CPM is that of variability of activity duration and points of convergence. Many, if not all, PERT/CPM networks have points where two (or more) activities must be completed before a third activity may begin. Assume activity times follow a beta distribution. In Fig. 2-3 Problem 1, activities A and B must be completed before activity C can begin. Since the expected duration of both A and B is 4 periods [E(A or B) = (2 + 4 × 4 + 6)/6 = 4], typical PERT/CPM planning would calculate that C will begin in period 4. However, if all possible combinations of the durations of both A and B are enumerated, the expected completion date of both A and B is 4.56 periods. The ultimate cause of the delay of activity C is the intersection of activities A and B (convergent point) when activity duration variability exists. With statistical fluctuations, convergent point calculations of start and finish times are incorrect.
Cause: Network conventions require that all paths converge to one end node.
Cause: Projects consist of dependent sequential activities, parallel paths, and convergent points.
Cause: Murphy exists.
Cause: PERT/CPM does not protect against Murphy.
These are addressed by Guideline VIII.
FIGURE 2-3 Eight problems with PERT/CPM management of single projects identified by Pittman (1994).
Problem 2: High Variability on a Non-Critical Path Figure 2-3 Problem 2 shows a simple PERT/CPM project with two paths. In typical PERT/CPM project management, the expected duration of each activity (assuming a beta distribution) is a simple point estimate based on the optimistic, most likely, and pessimistic estimates of activity duration. The expected duration of an activity is given by E(A) of an activity.
E(A) = (5 + 4 × 6 + 7)/ 6 = 6
E(B) = (3 + 4 × 4 + 5)/ 6 = 4
E(C) = (4 + 4 × 5 + 6)/ 6 = 5
E(D) = (1 + 4 × 4 + 7)/ 6 = 4
The upper path (C-D) would be expected to take 9 periods, and the lower path (A-B) would be expected to take 10 periods. The PERT/CPM critical