Consider a quality characteristic with a target of 100.00 [[Micrometre|μm]] and upper and lower specification limits of 106.00 μm and 94.00 μm, respectively. If, after carefully monitoring the process for a while, it appears that the process is out of control and producing output unpredictably (as depicted in the [[run chart]] below), weone can't meaningfully estimate its mean and standard deviation. In the example below, the process mean appears to drift upward, settle for a while, and then drift downward.
[[File:ProcessPerformanceExample.svg]]
If <MATH>\hat{\mu}</MATH> and <MATH>\hat{\sigma}</MATH> are estimated to be 99.61 μm and 1.84 μm, respectively, then
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The fact thatThat the process mean appears to be unstable is reflected in the relatively low values for P<SUB>p</SUB> and P<SUB>pk</SUB>. The process is producing a significant number of defectives, and, until the [[Common-cause and special-cause|cause]] of the unstable process mean is identified and eliminated, weone really can't meaningfully quantify how this process will perform.
