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11.6. Common Mistakes in Scheduling

Applies to:
@RISK 6.x/7.x, Professional and Industrial Editions
@RISK for Project 4.x

Do you have any guidelines for best practices in scheduling? What are some pitfalls I should avoid?

The attached slide presentation by one of our consultants shows some of the issues. This article presents a summary, for quick reference.

Most importantly, avoid dangling activities. You create a dangling activity when changes in a predecessor, such as longer duration, aren't properly transmitted to the successor. One way to do this is with Start-to-Start constraints or Finish-to-Finish constraints: if an earlier task runs long, the later task still starts or finishes on the original date. With dangling activities, you can't trust the dates, float, or critical path, and risks don't have their proper effect on the schedule.

One solution to this is Finish-to-Start constraints. These are best where two tasks really can't be done in parallel, but one must finish before the other one can start. However, if your tasks really can run in parallel, you can still prevent them from dangling by using Start-to-Start and Finish-to-Finish constraints. You need to create three milestones — for the start of the first task, and one for the end of each task — to persuade Microsoft Project to accept both constraints on the same pair of tasks; see pages 7 and 8 of the attached slides.

Another common mistake is activities with no predecessors or no successors. Every activity, except the first and the last, must have at least one Finish-to-Start or Start-to-Start predecessor relationship and one Finish-to-Start or Finish-to-Finish successor relationship, like this:

Predecessor Task → F-S or S-S → This Activity → F-S or F-F → Successor

Be wary of "Must Finish on" constraints on important finish dates. These can frustrate risk analysis of the very items you care about. You'll sometimes get messages from Project to the effect that a scheduling conflict prevents finishing this task in time. Pay heed to those messages, and don't ignore them or turn them off.

See also: How Are Tasks Scheduled in a Project?

Last edited: 2016-09-15


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