Do you have enough people to do the work in the time scheduled?
Now we come to another problem. You might have enough individuals to work each task in phases, but what happens when you schedule tasks in parallel? Do you still have enough resources?

Here is an example of parallel network shown in the PDM format. Task duration estimated by the team member (in days) is the number above each activity. The number of people needed to complete each activity is shown below each node. Functional managers supplying the manpower want to support the project, but they only have four people available. The sponsor has said the project should be completed in eight days. We are bounded by fixed constraints – or are we?

What tasks are on the critical path in this project?

There are two: A-D-F, and B-E-F. Each are 8 days. Only the A-C path has slack: 4 days.