Most of the literature in project scheduling with uncertain activity times advocates the use of the beta distribution for activity times. This is a continuous distribution with four parameters: a minimum, a maximum, and two parameters 1 and 2 that control the shape of the distribution. (@RISK calls this the generalized beta, or BetaGeneral, distribution because the “standard” beta distribution has minimum 0 and maximum 1.) It turns out that the PERT distribution we have been using is a special case of the generalized beta distribution. However, it has only three parameters:

a minimum, a most likely value, and a maximum.

To understand this better, try the following. Click on the Windows Start button, Programs, Palisade Decision Tools, Online Manuals, and finally Distribution Function Summary to open a PDF help file. Click on its bookmark tab to see a list of distribution functions and click on the PERT (Beta) bookmark. There you can see how the 1 and 2 parameters are found from the minimum, most likely, and maximum parameters of the PERT distribution. Specifically, they are chosen so that the mean of the PERT distribution is (min  4 ml

 max)6, where ml is the most likely value. You can also click on the Beta (Generalized) bookmark to see its properties. Now use this information from online help as follows. Suppose the parameters of a PERT distribution are 4, 5, and