Opinion on the use of constraints in a schedule?

What is your opinion on the use of constraints in a schedule? What would you consider to be a best practice? What do you think needs to be avoided? Please tell me how and when you use mandatory start and finish constraints, and all those in between. Feel free to tell me where the use of constraints impacted your critical path analysis.

actually constraints are very easy to find in project to the point I do not necessarily see a problem with the use of constraints; after all it is just one more tool in the schedulers tool box. I certainly would not deprove them of the ability to use it. What would interest me more is the reasons/conditions that people feel it is necessary. for example: 1) generally only appropriate with it is out of the control of the project (airline schedules) 2) when missing the data will have a detrimental effect on the schedule or cost also If it is absolutely necessary to have a constraint, then would it be appropriate to perform a risk analysis (to include accept the risk, treatment, reduction, transfer, etc.). just a thought

A caution on constraints: Normally I avoid their use since they make it very difficult to accurately determine the correct critical path in the schedule. And they also can make a mess out of any resource loading assumptions you may have originally built into the schedule. As Jon stated, a risk analysis is critical to understand just what the constraint is representing, and how to mitigate that risk. Lets take a weather example, I would suggest having a risk register that covers this ("IF the concrete is not poured BY November 15th, THEN poor curing due to low temperatures will require additional inspections, etc."). Good PM practice is to identify and track risks throughout the project timeframe. Putting artificial constraints into the project can hide a growing risk.

The use of constraints is one of the most hotly debated subjects among schedulers, although the prepoderance of opinions are in opposition to their use. As has been noted, the use of constraints can convolute a schedule and make it difficult, if not impossible, to trace the critical path(s). On the other hand, Finish-No-Later-Than date constraints are the mechanism by which we establish Late Finish dates for majot blocks of work, and are thus the springboard for Backward Pass calculations that give us our much-needed Late Dates which, in turn, give us the very Total Float values we use to monitor, evaluate, and priorotize project activity. What I have always found (surprisingly) missing from such discussions is the idea of including as many FNLT constraints as are warranted and then TURNING THEM ALL OFF, AND THEN ON, ONE CONSTRAINT AT A TIME. By turning them all OFF, one can easily trace the critical path to the projects completion. Then by turning on ONE constraint at a time, one can trace the critical path to EACH individual intermediate constraint. This approach yields the best of both worlds. One can ascertain, with relative accuracy, which activities DRIVE which constraints, and with how much influence. On the other hand, with all but the last one (final activity) turned off, the schedules Total Float values (and inferred Critical Path) are not obliterated,

Finish No Later than Constraint is an important one on the Project. Case Scenario High Rise Structure - 40+ Storeys with 7 Podiums and 3 Basements to be completed in 30 Months Time. Intermediate Milestone - Clients Offices : 7th Podium Floor to be completed and Handed over by the 14th month. This Milestone on the Project CPM should be depicted by Late Finish Constraint and not FS + Lag. Reason being any delays in the handover of the Client Office Floor does not necessarily imply delay on the project completion. To check the Constraint List on a Project CPM, the Schedule Log would be the best place. ----------------------------------------------------------------------------------------------------------------------------------------------------

We typically accept constraints but with conditions. A documented justification my be attached to the relevant tasks as a note. That would include a narrative / reason for the constraints, its supporting info or document e.g. a start manufacture, we would expect a document from the manufacturer with that date. etc. Constraints that emposed to Draw the schedule are not acceptable.