Graphical Evaluation and Review Technique

<h2 id="general-gert-rules">General GERT rules</h2>
<p>GERT uses activity-on-arrow notation only. That means that each activity is described on arrow. The nodes are used to connect activities, but also to determine type and conditions of relations between them.
</p><p>Each task has two parameters: duration and probability of appearance.
</p><p>There are three logical operators in GERT which concern activities incoming to the node:
</p>
<ul><li>XOR - alternative (only one path possible)</li>
<li>OR - alternative (one or more paths can be performed)</li>
<li>AND - all paths have to be performed</li></ul>
<p>The most common is AND, which means that every incoming activity has to happen before the outcoming one start.
</p><p>There are also two types of relations that concern activities outcoming from the node:
</p>
<ul><li>deterministic - every outcoming activity has probability equal to 1, which means that everyone will be performed</li>
<li>probabilistic - each outcoming activity has some probability of appearance.</li></ul>
<h2 id="gert-vs-other-network-diagrams">GERT vs. other network diagrams</h2>
<p>It's much less popular than PERT (Program Evaluation and Review Technique) or CPM (Critical Path Method) however, it allows some of those methods limitations to be addressed. The main limitations of PERT or CPM  that might be addressed by GERT are:
</p>
<ul><li>there are no alternative paths - all activities have to be performed</li>
<li>there are no loops - it is not possible to repeat activity, next similar activity has to be added</li>
<li>there are no decisions in the diagram - no and, or, xor options which help to choose proper path</li>
<li>there is no scaling - it is not possible to replace some detailed group of activities with the one summary task.</li></ul>
<p>GERT can help address these but at the cost of more sophisticated diagrams.
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Pritsker, A. A. B. (April 1966). "GERT: Graphical Evaluation and Review Technique" (PDF). RM-4973-NASA. National Aeronautics and Space Administration under Contract No. NASr-21. Retrieved 2006-12-05. <a href="/wiki/Alan_Pritsker" target="_blank">/wiki/Alan_Pritsker</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Modeling and Analysis Using Q-GERT Networks A. Alan B. Pritsker, 2nd Edition, Wiley, 1979 ISBN 0-470-26648-1 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

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Graphical Evaluation and Review Technique