Has anyone seen any sample questions that deal with calculating the PROJECT STANDARD DEVIATION?

After you calculate EAD (PERT), activity standard deviation & activity variance for each activity in the critical path my understanding is the following:

1. You add your EAD's (PERT) to calculate PROJECT DURATION
2. You add your activity variances to calculate PROJECT VARIANCE
3. You take the square root of the PROJECT VARIANCE to obtain the PROJECT STANDARD DEVIATION