Is Your Process Being Adjusted Too Frequently?

摘要

My consulting experience with manufacturing/engineering groups has revealed that virtually all of them have one or more hand-operated processes with someone in charge of keeping the process "on target." This has invariably led to over-adjusting, because: a) most of the time there is no control chart in place to guide them; and b) the operators (engineers or technicians) in charge of monitoring the processes inherently feel that, unless they are continually adjusting the processes, they are not doing the job they are paid to do. To be specific, let us suppose that a process output is designed to be 100. An operator measures the output and finds it to be 97. The process is adjusted by increasing its output by 3 units to correct the deficiency. This procedure is carried out from time to time. (It is related to "rule 2" of the "funnel experiment" described by Deming (1986, p. 327)). It is "obviously" a sensible approach. Unfortunately, as we shall see, it is often not a sensible approach at all (see MacGregor (1990) for situations in continuous process industries where frequent adjustment may be called for).