The reason is simple: Cpk is a prediction, and one can only predict something that is stable. And process performance indices should only be used when statistical control cannot be evaluated.
原因很简单：Cpk只是一个预测，只能对稳定的东西作出预测，而过程表现PpK则应该用在还没有进行管制的过程。
The error of the model is small and its performance is stable. The problem that error increases with the lapse of time existed in common fitting prediction has been overcome.
该模型误差小且稳定，克服了一般拟合预测中误差随时间推移增大的缺点。
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