Calculating slack time
Slack time is the total time a task can be delayed without causing additional delays and without affecting the planned end date of the project.
A project starts from 1 January with the following WBS items:
• WBS item 1 with a 10-day effort
• WBS item 2 with a 10-day effort
• WBS item 3 with a one-day effort
WBS item 2 has a dependency on WBS item 1. WBS item 1 must be completed before starting WBS item2.
The planned dates for these items are:
• WBS item 1: starts January 1, ends January 10
• WBS item 2: starts January 11, ends January 20
• WBS item 3: starts January 1, ends January 1
If WBS item 1 is delayed, the effort takes 11 days and the end date of WBS item 1 will be moved to January 11. As a result, WBS item 2 is also delayed. This also affects the end date of the project. The project is delayed and ends on January 21.
This means that WBS item 1 and WBS item 2 are both on the critical path: if one of them is delayed the project will also be delayed.
WBS item 3 has 20 days slack time, because it has no constraints or dependencies. It can be delayed for 20 days without influencing the end date of the project.
WBS items with a 'Must Start On' constraint are always part of the critical path too, even if they do not influence the end date. This is because items with a 'Must Start On' constraint have a very strict deadline (they MUST be executed at that time), so they must always have the attention of the person planning the project.