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CPA and ’normal’ disrtribution

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David Bordoli
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Joined: 8 Apr 2002
Posts: 416
Hello

A question, and sorry at this stage it may seem a little ‘off topic’ but if anyone knows the answer I guess it will be you risk experts.

It is generally accepted that the duration of a single activity will have a positively skewed probability distribution. Is the result of a critical path analysis also skewed or will it be normal as a result of the Central Limits Theorem coming into play? That states, basically, that the means of successive random samples taken from a population will be normally distributed whatever the underlying parent distribution.

Regards

David

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Philip Rawlings
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Joined: 16 Nov 2004
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David
The results of a risk analysis of a simple set of activities in series will ’tend’ towards a normal distribution as you state. BUT the tendency is lessened if the distributions are correlated together (depending on the degree of correlation), also depends on the shape of the network.

For a simple set of activities in parallel, you will probably get a left skewed distribution due to nodal/merger bias.

A real netwoek, of course, is more complex with a mix of parallel, series activities and some correlation so the effect is muted.