The difference between PERT and CPM

Screenshot 2023 05 08 at 10.58.47

CPM (Critical Path Method) is the idea of finding the critical path = the longest path = how long the project must take, but putting all of the dependences into a “network diagram”, adding up the various possible paths, and finding the longest one.  You can use either activity on arrow or activity on node for this.  Both will tell you right answer every time.   

A bit more about these types of network diagram: there’s “activity on node” where you put the time duration estimates in rectangles and join them up with dependency arrows. And there’s “activity on arrow” (less good in my opinion, more confusing) where you put the events in circles and you put your duration times on the arrows, so it’s the arrows that take the time rather than the circles.   

Meanwhile…. PERT is a way of estimating durations (and one I don’t really like, by the way) where you take 1x the lowest estimate, 4x the middle estimate, and 1x the highest estimate, and divide by 6. 

You can use PERT-derived times for either of the diagrams, activity on arrow or activity on node, though it was originally associated with activity on arrow (hence asymmetric on my graphic) – and you can also use normal “average” estimates for either diagram. 

It’s all very confusing, and in a way it doesn’t matter – just forget PERT, use averages and add on contingency to the total later.  Then use Activity on Node, probably with real or virtual post-its, and then make it all into a Gantt chart based around the critical path – everything will be just fine. ….but I just wanted to get it really really clear in my head and hopefully yours!

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