Simple priority rule combinations. An approach to improve both flow time and tardiness

Prior research indicates that no single priority rule displays superior performance on both flow time and tardiness measures. While the ‘shortest processing time’ rule does minimize the mean flow time, it performs poorly on the tardiness criterion. This study focuses on the same issue, whether both tardiness and flow time measures can be improved at the same time by combining simple priority rules. A total of 64 combination rules were developed by concurrently deploying four simple priority rules in all possible combinations in a simulated, three-stage serial manufacturing shop. The performance of the combinations was analysed and compared using three criteria, the mean flow time, mean tardiness, and the percentage of tardy jobs, under two shop load conditions. The results indicate that combinations using the ‘shortest processing time’ and ‘earliest due date’ rules produce excellent results on both tardiness and percentage of tardy jobs criteria without substantially sacrificing flow time. In addition, the shop load factor appears to have little effect upon the performance of these combinations.