Reexamination of processing time uncertainty

Processing times are difficult to accurately estimate in industries where custom-designed products are dominant. When schedulers are unable to make accurate estimates, what is the effect of estimation errors on the performance of scheduling policies? Processing time uncertainty (PTU) has become a focus of researchers in the production scheduling area. However, previous operational definition of PTU confounds the estimation error with the variation of actual processing time. In order to determine the effect of estimation errors, a thorough reexamination of prior studies becomes necessary. The results from our simulation experiments show that the estimation error has only a trivial effect on the performance of scheduling policies, and imply that the variation of actual processing time has a more important effect than the estimation error of processing time. The results from our simulation experiment have been verified by probabilistic analysis and Little's law.