Use of Hard Constraints

“But, over time, we realized that there were just too many constraints, and for ordinary citizens to be the change agent was not that easy.”

- Atishi

In this metric, DCMA is looking for inappropriate use (overuse) of hard constraints.

What are hard constraints?

A hard constraint is a rule imposed on the calculation of start and finish dates that interferes with standard calculations based on relationships. Hard constraints are more rigid than soft constraints. There are differences between Microsoft Project and Primavera, so we’ll look at them separately.

Hard constraints in Microsoft Project

In Microsoft Project, there are four kinds of hard constraints:

  • Finish No Later Than

  • Start No Later Than

  • Must Start On

  • Must Finish On

The common theme here is that hard constraints prevent start or finish dates from moving forward in time. Microsoft Project’s other constraints are more flexible and are sometimes called soft constraints:

  • As Soon As Possible

  • As Late As Possible

  • Start No Earlier Than

  • Finish No Earlier Than

The latter two can prevent a date from moving back in time.

Hard constraints in Primavera

In Primavera P6, hard constraints are easily defined as the “mandatory” constraints. An activity with a mandatory start or finish date is now anchored to that date. Everything else is considered to be a soft constraint.

Why are hard constraints bad?

A critical path is like a toy train set. When you push on one end, the other end should move. A hard constraint is like you’ve glued one of the cars to the track. Push on the train, and it won’t move.

Ideally, you want start and finish dates to be determined by the relationships between activities and not by inflexible rules that override those relationships. You want your schedule network to flow naturally forward in time, and constraints are obstacles to that natural flow.

That said, sometimes hard constraints are appropriate and necessary. If you are planning the opening ceremony for the Summer Olympics, there are hundreds of parties (like TV networks) that are expecting it to start at a specific time; an early or late start is not a viable option. DCMA is looking for hard constraints that are misused.

The threshold here is zero, so expect any hard constraints in your schedule to receive scrutiny and be prepared to justify their use in each case.

Next steps

Where hard constraints are used, investigate and learn the true reason for the constraint. Ask the following questions:

  • Why was the constraint added?

  • Is there a legitimate reason for the constraint?

  • What would the effect be of moving the constraint?

  • What would happen if the finish date of this task slipped?

14 Point Analyzer