THE SPT PRIORITY SEQUENCE RULE - THE ILLUSION OF EFFICIENCY AND THE REALITY OF BOTTLENECKS

Discusses the most widely accepted operations principle in the job shop working environment as the shortest processing time (SPT) job priority sequencing rule. It can be demonstrated that use of average job flow time as the measure of SPT efficiency in job flow scheduling introduces a mathematical artefact which can bias results in favour of SPT without adding any real efficiency. Average job flow time with SPT priority sequencing creates the illusion of efficiency. A simple simulation model is used to demonstrate that even though average job flow time is clearly and significantly shorter with SPT it can mask under-utilization of capacity-constrained work stations - bottlenecks - present in the flow. Giving priority to those jobs which fill capacity at a bottleneck significantly improves capacity utilization at the bottleneck and reduces the amount of work which awaits capacity at the conclusion of the work schedule.