A comparative simulation study was performed to investigate the effect of uncertainty or unreliability in job processing times on designing schedules in a pure job shop environment. Five performance measures were employed in an effort to examine the effect of variations between estimated and actual processing times. These variations are common in job shop systems where estimations are used to set processing times. It was hypothesized that a variation has an effect on relative system performance. In addition, three priority rules, namely, FIFO, SOT and EDD, were studied to determine the best rule for controlling the shop in situations of unreliable processing times data. The percentage variation in processing times was clearly the dominant variable for most of the performance measures. A 60% variation in estimated processing times appeared to be the maximum level under which the mean flowtime, mean tardiness and percent of late jobs performance measures were insensitive. The simulation results indicated that the makespan and average shop utilization performance measures were insensitive to reliability of input information about processing times for up to a 100%. The performance of FIFO and EDD priority rules suffered drastically while SOT was the least sensitive to the reliability of processing times information. / M.S.