Program evaluation and review technique
The program evaluation and review technique ( PERT ; also called event node representation ) is an event- oriented network plan technique .
history
The technology was developed in 1958 as part of the Polaris project . The particular difficulty of the Polaris project was that both research and development as well as the production of the components, which had never been manufactured before, had to be outsourced to suppliers. Neither costs nor time requirements could be estimated even approximately exactly. The deadlines could only be based on probability. Each supplier was therefore asked to estimate the time required. It is estimated that PERT was able to complete the Polaris rocket two years, 45% earlier. A related procedure is the Graphical Evaluation and Review Technique (GERT) , with the help of which stochastic execution probabilities for individual processes can be mapped.
PERT and CPM
PERT is very similar to the developed almost simultaneously Critical Path Method ( critical path method, CPM ), but used instead of the most common duration the expected average duration of tasks as a basis for the calculation in the network planning. CPM is used for standardizable projects whose processes are largely known from the experience of other projects and for which there is comparatively little uncertainty regarding the time estimate. PERT is used for projects with high uncertainty and little experience.
PERT specialty
The time in which a process can be carried out is not estimated as a scalar quantity with PERT . Rather, a probability distribution is assumed. PERT uses the beta distribution as a basis, which has proven itself in practice for this purpose. As a rule, the minimum or optimistic estimated duration d min , the most frequent (estimated to the best of our knowledge) duration d norm , and the maximum or pessimistic estimated duration d max are required for the effort estimate of a process (so-called three-time estimation) . The mean duration to be used for the process then results from the beta distribution:
With PERT it is also possible to specify the structure “not unambiguously” ( stochastically ). The network plan is evaluated with the help of decision nodes, with the following node types: on the input side AND , OR , EXCLUSIVE-OR , on the output side deterministic and stochastic .