Evaluating simple scheduling rules in a mixed shop environment

The layout and flow of real manufacturing shops are most often neither pure job-shop nor pure flow-shop. This paper presents the results of a simulation-based study in which some of the most common and simple scheduling rules are tested in mixed shops containing jobs with both job-shop and flow-shop routing. Specifically, the rules that are used are first in first out, shortest processing time, modified shortest imminent operation, non-decreasing slack, and modified slack. Two performance measures are considered, a modified mean flow time to assess the amount of time that jobs spend waiting for processing and the cost of earliness and tardiness to evaluate how closely the rules complete jobs relative to their due date. The data are analyzed using both graphical and hypothesis tests so that not only conclusions based on general trends could be drawn, but also assessed wih statistical rigor. Using these techniques, the results are somewhat mixed but, in general, indicate that the shortest processing time rule is preferred when the mean flow time is the performance measure and modified slack is preferred when cost is the performance measure. By adding additional insight from the actual simulation runs to these conclusions, we recommend that practitioners can likely get good performance by using either of these rules as the basis for a heuristic that adds logic to avoid extreme conditions.