|CPM uses activity oriented network.||PERT uses event oriented Network.|
|Durations of activity may be estimated with a fair degree of accuracy.||Estimate of time for activities are not so accurate and definite.|
|It is used extensively in construction projects.||It is used mostly in research and development projects, particularly projects of non-repetitive nature.|
|Deterministic concept is used.||Probabilistic model concept is used.|
|CPM can control both time and cost when planning.||PERT is basically a tool for planning.|
|In CPM, cost optimization is given prime importance. The time for the completion of the project depends upon cost optimization. The cost is not directly proportioned to time. Thus, cost is the controlling factor.||In PERT, it is assumed that cost varies directly,with time. Attention is therefore given to minimize the time so that minimum,cost results. Thus in PERT, time is the controlling factor.|
Graphical Evaluation and Review Technique (GERT) is a network analysis technique used in project management that allows probabilistic treatment of both network logic and estimation of activity duration. GERT is mainly used on project activities that are only performed in part, as well as those activities that may be performed more than once (loop).
How GERT Overcomes Limitations of PERT/CPM:
GERT allows additional branching features not provided by CPM or PERT.
GERT allows both deterministic and probabilistic branching.
In GERT, not all immediate predecessor activities must be completed before activity is started.
GERT allows looping of activities to allow activities that need to be performed more than once.