Job sequencing rules determine the priority for processing jobs. They address scheduling problems that generally lead to inefficient performance in assigning jobs and completing them. Of central importance is job flow time, which is the amount of time a job spends in a shop from order placement to its completion and release. The average time taken to complete jobs is one way of measuring a job shop's performance.
Some shops sequence jobs by earliest due date. Sometimes called due date assignment, it places the highest priority on processing jobs with the earliest due dates. Job shop quality performance can be measured by the number of late jobs, the average tardiness across late jobs or the average tardiness across all jobs.
The longest processing time approach to job sequencing assigns the highest priority to jobs with the longest processing time. By scheduling the longest job first, schedulers are able to reduce the number of outstanding long jobs at the end of the job schedule. The completion time for the last jobs—which are among the shortest in length—is also reduced.
Another method of job sequencing based on job completion time, the shortest processing time method assigns jobs with the shortest processing time first. Like the LPT scheduling method, SPT requires an initial time estimation for each job. SPT reduces average flow time for jobs, according to Kenneth R. Baker in "Management Science."
Many shope employ the first-come, first-served job sequencing method, which processes job orders in the order of their arrivel at the production center. Arrival time is the determinative factor in this scheduling rule, which is also referred to as first-in, first-out. It is sometimes described as the simplest job sequencing rule. For example, unlike SPT and LPT scheduling methods, no time estimation is required for FCFS scheduling.
The preferred customer order approach prioritizes jobs from preferred customers, and can be useful tool for building customer loyalty.