استفاده از بافرها در مدیریت پروژه : سبک سنگین کردن بین ثبات و تغییر

During execution projects may be subject to considerable uncertainty, which may lead to numerous schedule disruptions. Recent research efforts have focused on the generation of robust project baseline schedules that are protected against possible disruptions that may occur during schedule execution. The fundamental research issue we address in this paper is the potential trade-off between the quality robustness (measured in terms of project duration) and solution robustness (stability, measured in terms of the deviation between the planned and realized start times of the projected schedule). We provide an extensive analysis of the results of a simulation experiment set up to investigate whether it is beneficial to concentrate safety time in project and feeding buffers, or whether it is preferable to insert time buffers that are scattered throughout the baseline project schedule in order to maximize schedule stability.

The basic objective of this paper was to research the potential trade-off between solution robustness (stability) and quality robustness (makespan performance) using three heuristics as the vehicle of analysis: the original CC/BM and modified CC/BM approaches, being heuristics that aim at makespan performance, and ADFF, being a heuristic that aims at stability. The main conclusion of the comparative study is that the expected difference in makespan performance between makespan protecting schedules and solution robust schedules tends to disappear for some projects. Where this is the case, a solution robust schedule will most likely be preferred because of the considerably lower stability cost. ADFF, for example, seems to be particularly interesting for projects for which a heavy weight is given to timely completion, i.e. for which quality robustness really matters. Paradoxically, the pioneers of CC management focus on due date performance, while their approach seems to be hard to defend when the timely realization of the projects is deemed important. The fact that stability costs of the real activities are relatively small does not justify them to be ignored. Indeed, solution robust scheduling techniques ascribe accordingly little attention to intermediate activities, which will also result in a projected schedule with a large project buffer, but opposing to makespan protecting schedules, stability is not ignored and is substantially better. CC/BM-based schedules will in general suffer less from the solution/quality robustness trade-off when the project has a larger number of activities or higher order strength. An important lesson to be learned from this paper is that project managers should be aware of the trade-off between stability and makespan performance. The buffering strategy should be chosen with an eye on the characteristics of the project to be scheduled.