Example of PERT analysis
Wednesday, April 07, 2010
6:36 PM
Suppose we have an empirical distribution as follows: 
0-10
50 instances
p=50/100
10-20
40 instances
p=40/100
20-30
10 instances
p=10/100
Caveat: 
distributions change over time. 
sensitive to time of day and external load. 
sensitive to external events. 
Call this d
If 0<=d<10, d is not critical, delay is 35 with p=1/2
If 10<=d<20, d is not critical, delay is 35, p=4/10
if 20<=d<30, d is critical, delay is 35-45, p=1/10
SO
We can compute the expected value, which is the sum of value * Prob(value), where the value is an interval 
= (35, 35)*9/10 + (35,45)*1/10