Effects of Setup and Processing Time Reductions on WIP in the JIT Production Systems
质疑Sarkar和Zangwill关于减少准备和加工时间会增加在制品库存的结论，指出在JIT系统中，准备时间减少不一定导致排队无限增长，且减少幅度和方差水平共同决定等待时间变化，有助于规划可行的准备时间削减项目。
Sarkar and Zangwill (S-Z, [Sarkar, D., W. I. Zangwill. 1991. Variance effects in cyclic production systems. Management Sci. 37(4, April) 444–453.]) have shown that in the presence of variance, setup and processing time reductions can have “detrimental” effects on the work-in-process (WIP) inventory in both push and pull (JIT) systems. In this note, we point out that the merits of such a warning for JIT production systems are questionable. If we treat a setup as a PERT network, it is difficult to accept S-Z claim that waiting queues can grow without bound when setup time is reduced. Furthermore, we show that the amount of setup cut, in addition to the level of variance, can determine whether waiting time grows or not. This finding can help in planning a viable setup reduction project. Additionally, we use S-Z example to show that, even when the variances are not reduced proportionately, the expected waiting time does not necessarily increase.