Priorities

The FAST system allows users to specify any priority value from 1 to 999.  Many other systems ignore priorities and some others mistake priorities for predecessors.  Those systems ask the user to specify a “Priority” and the system uses the number supplied as the timeline sequence, earliest to latest.  By contrast, the FAST system interprets the priority value to mean the importance that the activity  be on the schedule, once it is built.  The difference is significant.   In circumstances where time sequence isn’t necessary or the most important scheduling criterion, priority should not mean “sequence”.  For example, in scheduling operating rooms in a hospital, the priority is more associated with the urgency of the procedure; critically ill patients get priority over elective surgery patients.  In a job shop, jobs with the bigest demand or the ones that are hardest to schedule may have priority over other jobs.

The FAST system allows you to use priority as it is intended.  It does not mean time ordering.  Higher priority activities are placed on the timeline earlier in the scheduling process, but not necessarily  at an earlier start time.  The higher priority activities are placed on the timeline at start times that are dictated by the decision rules being employed.  Therefore, start times may be those when the activities fit theb best according to  resource utilization considerations, or when costs are minimized, or when it’s best from a “just-in-time” standpoint.  We will describe the objective function capabilities of the FAST system in a subsequent blog posting.

Priorities affect the processing sequence, but not necessarily the time sequence in the final schedule.  This removes another serious limitation of other schedulers and improves the quality of the schedules produced.  In fact, this feature allows the FAST scheduler to find schedules that are feasible but that other schedulers cannot find at all!

How do priorities work when there are predecessor relationships among the activities?  This excellent question will be answered in the next posting on predecessors and precedence networks.  Be sure to read the next post!