Mandatory Constraints

I am not a great supporter of “Scheduling Rules”, but I do feel it is important to understand how constraints work and then it is possible to explain to clients why the constraints are selected and what effect they have on the schedule.

During the forward and backward pass the Early and Late dates are calculated and in P3, it is important to understand this process to understand how constraint operate:

Early Start and Finish constraints operate on the Early date calculations.

Late Start and Finish constraints operate on the Late date calculations.

Start and Expected finish date constraints operate on both Early and Late dates and are effectively applying Early and Late constraint at the same time but still allow float calculation to pass through them.

A mandatory constraint operates differently to other constraints. Mandatory constraints do not allow float calculations to pass through them. A mandatory constraint in the middle of a schedule will effectively break the schedule in too two separate forward and backward pass calculations.

Zero Free float makes the activity As Late as Possible by absorbing all the Free Float, in MSP an As Late as Possible constraint absorbs all the Total Float and delays all the successor activities, P3 only delays the task with the constraint.

Zero Total Float sets the late dates to equal the early dates so an activity with this constraint will not display any float.

The final constraint that may be applied to a schedule is a Project Must Finish by date which imposes a Late Finish on all project tasks and float is calculated to this date when set, otherwise float is calculated to the last activity in the Critical Path.

I suggest that you play with a small schedule and get to understand how the constraints work and then apply the most suitable constraint to the environment and explain with authority why a particular constraint has been used.

Regards
Paul E Harris
www.eh.com.au

Whew! Well said, Paul!

Jorge