Жоба басқарудағы PERT әдісі

Statistical tool used in project management

The program evaluation and review technique (PERT) is a statistical tool used in project management, which was designed to analyze and represent the tasks involved in completing a given project. PERT was originally developed by Charles E. Clark for the United States Navy in 1958; it is commonly used in conjunction with the Critical Path Method (CPM) which was also introduced in 1958.

PERT is a method of analyzing the tasks involved in completing a project, especially the time needed to complete each task, and to identify the minimum time needed to complete the total project. It incorporates uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. It is more event oriented than start and completion oriented, and is used more for projects where time is the major constraint rather than cost. It is applied to very large scale, one time, complex, non routine infrastructure projects, as well as R&D projects. PERT offers a management tool, which relies "on arrow and node diagrams of activities and events: arrows represent the activities or work necessary to reach the events or nodes that indicate each completed phase of the total project." It found applications throughout industry. An early example is the 1968 Winter Olympics in Grenoble which used PERT from 1965 until the opening of the 1968 Games. This project model was the first of its kind, a revival for the scientific management of Frederick Taylor and later refined by Henry Ford (Fordism). DuPont's CPM was invented at roughly the same time as PERT. Initially PERT stood for Program Evaluation Research Task, but by 1959 was renamed. and Phase 2. both primarily written by Charles F. Clark. Ten years after the introduction of PERT, the American librarian Maribeth Brennan compiled a selected bibliography with about 150 publications on PERT and CPM, all published between 1958 and 1968. For the subdivision of work units in PERT another tool was developed: the Work Breakdown Structure. The Work Breakdown Structure provides "a framework for complete networking, the Work Breakdown Structure was formally introduced as the first item of analysis in carrying out basic PERT/CPM."

The first step for scheduling the project is to determine the tasks that the project requires and the order in which they must be completed. The order may be easy to record for some tasks (e. g., when building a house, the land must be graded before the foundation can be laid) while difficult for others (there are two areas that need to be graded, but there are only enough bulldozers to do one). Additionally, the time estimates usually reflect the normal, non rushed time. Many times, the time required to execute the task can be reduced for an additional cost or a reduction in the quality.

In the following example there are seven tasks, labeled A through G. Some tasks can be done concurrently (A and B) while others cannot be done until their predecessor task is complete (C cannot begin until A is complete). Additionally, each task has three time estimates: the optimistic time estimate (o), the most likely or normal time estimate (m), and the pessimistic time estimate (p). The expected time (te) is computed using the formula (o + 4m + p) ÷ 6. Activity Predecessor Time estimates Expected time Opt. (o) Normal (m) Pess. (p) A — 2 4 6 4.00 B — 3 5 9 5.33 C A 4 5 7 5.17 D A 4 6 10 6.33 E B, C 4 5 7 5.17 F D 3 4 8 4.50 G E 3 5 8 5.17

Once this step is complete, one can draw a Gantt chart or a network diagram.

Depending upon the capabilities of the data input phase of the critical path algorithm, it may be possible to create a loop, such as A > B > C > A. This can cause simple algorithms to loop indefinitely. Although it is possible to "mark" nodes that have been visited, then clear the "marks" upon completion of the process, a far simpler mechanism involves computing the total of all activity durations. If an EF of more than the total is found, the computation should be terminated. It is worth saving the identities of the most recently visited dozen or so nodes to help identify the problem link.

PERT chart explicitly defines and makes visible dependencies (precedence relationships) between the work breakdown structure (commonly WBS) elements. PERT facilitates identification of the critical path and makes this visible. PERT facilitates identification of early start, late start, and slack for each activity. PERT provides for potentially reduced project duration due to better understanding of dependencies leading to improved overlapping of activities and tasks where feasible. The large amount of project data can be organized and presented in diagram for use in decision making. PERT can provide a probability of completing before a given time.

There can be potentially hundreds or thousands of activities and individual dependency relationships. PERT is not easy to scale down for smaller projects. The network charts tend to be large and unwieldy, requiring several pages to print and requiring specially sized paper. The lack of a timeframe on most PERT/CPM charts makes it harder to show status, although colours can help, e. g., specific colour for completed nodes.

During project execution a real life project will never execute exactly as it was planned due to uncertainty. This can be due to ambiguity resulting from subjective estimates that are prone to human errors or can be the result of variability arising from unexpected events or risks. The main reason that PERT may provide inaccurate information about the project completion time is due to this schedule uncertainty. This inaccuracy may be large enough to render such estimates as not helpful. One possible method to maximize solution robustness is to include safety in the baseline schedule in order to absorb disruptions. This is called proactive scheduling, however, allowing for every possible disruption would be very slow and couldn't be accommodated by the baseline schedule. A second approach, termed reactive scheduling, defines a procedure to react to disruptions that cannot be absorbed by the baseline schedule.