Program evaluation and review technique: Difference between revisions

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[[File:PERT Summary Report Phase 2, 1958.jpg|thumb|upright|''PERT Summary Report Phase 2'', 1958]]

Initially PERT stood for ''Program Evaluation Research Task,'' but by 1959 was already renamed.<ref name="MRCW 1959"/> It had been made public in 1958 in two publications of the U.S. Department of the Navy, entitled ''Program Evaluation Research Task, Summary Report, Phase 1.''<ref>U.S. Dept. of the Navy. ''[http://www.dtic.mil/dtic/tr/fulltext/u2/735902.pdf Program Evaluation Research Task, Summary Report, Phase 1].'' Washington, D.C., Government Printing Office, 1958.</ref> and ''Phase 2.''<ref>U.S. Dept. of the Navy. ''[https://catalog.hathitrust.org/Record/100954569 Program Evaluation Research Task, Summary Report, Phase 2].'' Washington, D.C., Government Printing Office, 1958.</ref> In a 1959 article in ''The American Statistician'' the main [[Willard Fazar]], Head of the Program Evaluation Branch, Special Projects Office, U.S. Navy, gave a detailed description of the main concepts of the PERT. He explained:

{{Quote|Through an electronic computer, the PERT technique processes data representing the major, finite accomplishments (events) essential to achieve end-objectives; the inter-dependence of those events; and [[Estimation (project management)|estimates]] of time and range of time necessary to complete each activity between two successive events. Such time expectations include estimates of "most likely time", "optimistic time", and "pessimistic time" for each activity. The technique is a management control tool that sizes up the outlook for meeting objectives on time; highlights danger signals requiring management decisions; reveals and defines both methodicalness and slack in the flow plan or the network of sequential activities that must be performed to meet objectives; compares current expectations with [[Schedule (project management)|scheduled]] completion dates and computes the probability for meeting scheduled dates; and simulates the effects of options for decision&nbsp;— before decision.<br />The concept of PERT was developed by an operations research team staffed with representatives from the Operations Research Department of [[Booz Allen Hamilton]]; the Evaluation Office of the [[Lockheed Martin Space Systems|Lockheed Missile Systems Division]]; and the Program Evaluation Branch, Special Projects Office, of the Department of the Navy.<ref name="SDFJWM 1959">[[Willard Fazar]] cited in: B. Ralph Stauber, H. M. Douty, Willard Fazar, Richard H. Jordan, William Weinfeld and Allen D. Manvel. "[https://www.jstor.org/stable/2682310 Federal Statistical Activities]." ''The American Statistician'' 13(2): 9-12 (Apr., 1959) , pp. 9-12</ref>}}

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=== Uncertainty in project scheduling ===

During project execution, however, 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 the anticipated disruptions. This is called ''proactive scheduling''. A pure proactive scheduling is a utopia; incorporating safety in a baseline schedule which allows for every possible disruption would lead to a baseline schedule with a very large make-span. A second approach, termed ''reactive scheduling'', consists of defining a procedure to react to disruptions that cannot be absorbed by the baseline schedule.