Total Slack quantifies the flexibility in your schedule. It is the number of working days that a path may slip or extend without impacting the project due date.
It is the difference between:
- The backward and forward passes.
- The late dates (the ‘Must Happen dates’) and the early dates (the ‘Can Happen’) dates.
- The time available for a path and the time needed for a path.
When Total Slack is calculated, if the value is:
- Positive – This is a good thing, because it means there is more time available than work. The Total Slack indicates how many days early our project is currently forecast to finish.
- Negative – This is a bad thing, because it means there is more work to do than time available. The Total Slack indicates how many days late our project is currently forecast to finish.
- Zero – This is also bad, because it means that no task in the path can slip or extend even one day without affecting the project end date.
In the CPM process, Total Slack is calculated for each task and milestone in the schedule model. However, Total Slack is actually a property of the path. If any task on a path uses part of the Total Slack, the Total Slack is reduced for all tasks on that path.
The formula to calculate Total Slack is Late Finish – Early Finish.
In the example below, the forward pass is performed starting at day 1, based on the Project Start Date. The backward pass is driven by the Must Finish On constraint at day 22.
The Total Slack values are calculated using the formula above:
Total Slack = Late Finish – Early Finish
As described in the previous section, when a Must Finish On constraint is assigned to a milestone, the Early Start, Early Finish, Late Start, and Late Finish dates for that milestone are ALL set to the Must Finish On Constraint Date (22).
The Total Slack on the Target Complete (the milestone with the MFO) is 0.
In Microsoft Project, this will always be true if the Total Slack on the critical path (which will include the Project Complete milestone which is the predecessor of the Target Complete) is either positive or 0.
However, if the Total Slack on the critical path is negative, Microsoft Project will show that same negative value in the Total Slack for the Target Complete task. The ES, EF, LS, and LF will all still be set to the MFO date, however the Total Slack will not be calculated in this case using LF – EF.
Total Slack on the milestone with the MFO assigned will always be negative or 0.
In the example below, Task A has been re-estimated to be 13 days, instead of 10 days. There is a change in the finish of Task A.
The forward pass is recalculated.
The backward pass doesn’t change (the due date is the same).
Therefore, the Total Slack is recalculated and reduced for all tasks and milestones in the path (except the milestone with the MFO which still has Total Slack = 0).
This situation would apply whether the change in the finish of Task A is caused by increase in the estimated Duration, an Actual Duration (from status on the task) that exceeded the original Duration, or a delay in the start date that then caused a delay in the finish with the same Duration.