Start and finish dates for activities in a network diagram are normally calculated from the forward pass and backward pass of critical path analysis. Sometimes it is necessary to take external influences into account. These could include material delivery dates, approval meeting dates, contractual delivery dates and so on.
Most scheduling tools accommodate a variety of types of imposed date such as:
- must start later than;
- must start on;
- must finish earlier than;
- must finish on.
These constraints can be applied to either the start or the finish of activities. Each can have an effect on the critical path.
In the first example, activity F cannot start until day 16 because of an external constraint. As a result the critical path begins at the start of that activity.
In the second example, activity F cannot finish later than day 16. This creates a problem with preceding activities and a path with negative float.
Sequences of activities with negative float are referred to as ‘supercritical’ or ‘hypercritical’ paths.