The use of cost‐based priorities in random and flow shops

Abstract

AbstractThis article evaluates the use of cost information for establishing priorities in a job shop. Most traditional research in the area of job shop scheduling uses only time‐oriented rules and performance criteria when in reality, minimizing total cost is the most appropriate objective.Utilizing a simulation model of randomly‐routed and flow job shops, several priority rules are evaluated on the basis of time performance, primarily on‐time completions, as well as on the basis of cost performance: the minimization of work‐in‐process (WIP) investment. The priority rules use either time information only, cost or value information only, or both types of information in a composite manner. An additional concept examined is that of the speed of a job relative to its due date. Speed is examined for different profitability classes of jobs: high, medium, and low. It is often important that high profit jobs are completed on time, while low profit jobs may not be as critical.The priority rules are simulated at four utilization levels and several replications are made to make statements about the statistical significance of the results. The results show that a time rule, such as critical ratio, performs poorly on cost measures, moves high and low profit jobs indiscriminately, and completes significantly fewer jobs on time at high utilization levels than the cost rules examined. One of the cost rules that utilizes the value added to a job is the best performer in terms of cost and does quite well in completing jobs on time.