Spider has a nice feature of probabilistic analysis of project completion date, cost etc. It is based on optimistic, pessimistic and most probable schedule scenarios.

It is clear, that probability to meet optimistic scenario is 0%, pessimistic is 100%. However the question is: how does it calculate probabilities for values in between optimistic and pessimistic?

In my understanding, to build correct finish date probability distribution, Spider should assume some probability distribution function for each of the parameters (e.g. durations) for which optimistic and pessimistic values are different from each other (e.g. beta-distribution or triangle distribution). Then use something like Monte Carlo analysis to calculate the entire schedule probability distribution. But something makes me believe, that Spider is not doing Monte Carlo analysis. So, how is the end schedule probability distribution calculated, does it just assume certain probability distribution function of finish date of entire project?

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