在画出项目活动网络图，准备寻找关键路径之前，必须先将每个在活动用时(Duration)进行估算，PMBOK 中提到的几种估算方法(PMBOK 2004 3rd P141-P142)中有一种叫做Three-Point Estimates，这其实就是PERT方法 - Program Evaluation and Review Technique. 这一方法最早产生于美国海军在研发 Polaris系统时。当时人们想出这种方法是因为这是一项从来没有做过的事情，没有任何数据来估算项目的时间。由此可见PERT适用于不确定条件下的时间估算。

PERT assumes a beta probability distribution (beta分布) for the time estimates. For a beta distribution, the expected time for each activity can be approximated using the following weighted average:

Expected time (期望) =  ( Optimistic  +  4 x Most likely  +  Pessimistic ) / 6

This expected time may be displayed on the network diagram.

To calculate the variance for each activity completion time, if three standard deviation times were selected for the optimistic and pessimistic times, then there are six standard deviations between them, so the variance is given by:

[ ( Pessimistic  -  Optimistic ) / 6 ]²

SD(标准差) = ( Pessimistic  -  Optimistic ) / 6

(from http://www.netmba.com/operations/project/pert/)