The disadvantages of constraints. If you have particular dates in mind for several activities in your schedule you may be tempted to insert constraints to fix the start or finish dates of these activities. Try to avoid this temptation, as constraints come with disadvantages.
Constraints are not forbidden in most scheduling guidelines, but their broad usage is discouraged. Item 12 of the Naval Facilities Command (NAVFAC) Initial Project Schedule (IPS) checklist says that all constraints must be contractually defined. So here we see that constraints are allowed by the NAVFAC checklist but restricted in a way to discourage their wide usage.
This article discusses the disadvantages of constraints in scheduling.
- One major problem with schedule constraints is that they tend to fragment the critical path. An intermittent or fragmented critical path or longest path is a show stopper. Multi-hundred million dollar projects have been suspended pending resolution of a longest path discontinuity. Modeling important dates in the life of the project in a way that does not fragment the critical path is a delicate process. This article “Scheduling Non-Contractual Important Project Task Dates” shows an example situation where a special date was defined and/or adhered to without the insertion of a constraint.
- Another major problem with constraints is that they are static. They are fixed dates in the schedule. Ideally you want dependencies to push tasks to the proper place on the timescale. This is not possible if the respective tasks are held in place with constraints. This is because constraints are stronger than dependencies. So the constraints will fix a task date and hold that date in place even though schedule updates and dependencies would say otherwise.
- Constraints make it more difficult to investigate schedule scenarios. This, again, is due to the static nature of constraints. Most likely, your constraint dates must be manually changed for each schedule scenario investigated. This makes investigating multiple schedule situations a tedious and laborious process.
- Mandatory hard constraints may violate network logic. The whole foundation of the precedence diagram or network logic becomes suspect when the schedule has mandatory hard constraints. In order to keep the mandatory constraint date fixed the dependency relationship is neglected and dishonored. For more on why to avoid mandatory hard constraints refer to the following article “Why You Should Avoid Mandatory Activity Constraints”.
Summary
Limit the use of constraints in scheduling your project. Constraints are a major issue when it comes to longest path discontinuities. Instead of one continuous longest path from project start to finish you have a fragmented critical path. Constraints also inhibit the dynamic quality of schedules, and should therefore, again, be limited.
Schedule updates and investigations become laborious when the schedule has many constraints. And mandatory hard constraints will hold their constraint dates, but violate associated network dependencies in the process of doing so. Limiting usage of constraints to rare situations is recommended good scheduling practice.
Again, contractually defined constraints are acceptable. It may require proficiency in the art of scheduling to define important dates in the life of the project and simultaneously avoid insertion of constraints. And if constraints are required, you also want to provide a reasonable explanation for the insertion of each constraint.