先说说PERT:
PERT虽然是常说常考的技术,但是在PMBOK2004里面却只字未提,实在是有点儿奇怪. 下面把PMBOK2000里面相关的叙述抄一下.

6.2.3 Output from Activity Sequencing

.1 Project network diagrams (PDM & ADM)

“A project network diagram is often referred to as a PERT chart. Historically PERT (Program Evaluation and Review Technique) was a specific type of network diagram.”

6.4.2 Tools and Techniques for Schedule Development

.1 Mathematical analysis. Mathematical analysis involves calculating theoretical early and late start and finish dates for all project activities without regard for any resource pool limitations. The resulting dates are not the schedule, but rather indicate the time period within which the activity could be scheduled given resource limits and other constraints. The most widely known mathematical analysis techniques are:

-          Critical Path Method (CPM) – calculates a single, deterministic early and late start and finish date for each activity based on specified, sequential network logic and a single duration estimate. The focus of CPM is calculating float to determine which activities have the least scheduling flexibility. The underlying CPM algorithms are often used in other types of mathematical analysis.

-          Graphical Evaluation and Review Technique (GERT) – allows for probabilistic treatment of both network logic and activity duration estimates (i.e., some activities may not be performed at all, some may be performed only in part, and others may be performed more than once).

-          Program Evaluation and Review Technique (PERT) – uses a weighted average duration estimate to calculate activity durations. Although there are surface differences, PERT differs from CPM primarily in that it uses the distribution’s mean (expected value) instead of the most likely estimate originally used in CPM. PERT itself is seldom used today.

从以上叙述中可以知道：

PERT技术属于项目时间管理知识领域，是基于项目网络图(PDM和ADM)的一种分析技术，用来在制定项目进度时，对存在高度不确定因素的活动的工期作估计，希望获得一个比较准确的估计，并且可以对工期进行概率计算。

对每个活动工期估计3个数-乐观的，最可能的，保守的，然后按Beta概率分布算出均值作为该活动的期望工期。Beta概率分布的公式是：

期望的活动工期 = (乐观的 + 4 x最可能的 + 保守的)/6

活动工期的方差 = ((乐观的 - 保守的)/6) ^ 2

仅仅有单个活动的期望工期是没有意义的，对于项目进度，需要了解的是关键路径(CP)的期望工期，即每个关键活动的期望工期之和。当采用三个时间估计（Beta概率分布）时，网络图中关键路径上所有活动的工期估计加起来可以得到一个总概率分布，由概率理论可知，这个总概率是正态概率分布，其曲线是以平均值为对称轴的钟形曲线。这个总概率分布的期望工期等于构成总分布的各项活动期望工期之和，其方差等于构成总分布的各项活动的方差之和；由此，我们可以得出总概率分布的标准差（SD）：

项目工期的标准差 = (关键活动方差之和) ^ (1/2)

对于正态分布，期望值两边1个标准差的范围内，曲线下面积约占总面积的68%；2个标准差范围内，曲线下面积约占总面积的95%；3个标准差的范围内，曲线下面积约占总面积的99%。因此我们可以知道，项目在期望工期完成的概率是50%，在(期望工期+1个标准差)时间内完成的概率是(50%+(68%/2))=84%，在(期望工期+2个标准差)时间内完成的概率是(50%+(95%/2))=97.5%，在(期望工期+3个标准差)时间内完成的概率是(50%+(99%/2))=99.5%。

例子在课堂讲义里都有了。