Statistics 简明教程
Statistics - Process Capability (Cp) & Process Performance (Pp)
Process Capability
过程能力可以定义为过程相对于其规格的可测量属性。它以过程能力指数 ${C_p}$ 表示。过程能力指数用于检查过程产生的输出的变异性，并将变异性与产品公差进行比较。 ${C_p}$ 由以下公式控制：
Formula
其中——
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${USL}$ = 上规格限。
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${LSL}$ = 下规格限。
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${\mu}$ = 过程的估计均值。
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${\sigma}$ = 过程的估计变异性，标准差。
过程能力指数 ${C_p}$ 的值越高，过程越好。
Example
考虑一辆汽车及其停车位的案例。车库尺寸表示规格限，而汽车定义为过程输出。此处，过程能力将说明汽车尺寸、车库尺寸以及汽车停放在车库中间有多远之间的关系。如果汽车尺寸比车库尺寸小很多，则可以轻松将汽车停入其中。如果汽车尺寸与车库尺寸相比非常小，则可以从中心任意距离停入。就过程控制而言，这种变化较小的过程允许轻松地将汽车停放在车库中，并满足客户的要求。让我们根据过程能力指数 ${C_p}$ 来考虑上述示例。
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${C_p = \frac{1}{2}}$ - 车库尺寸小于汽车，无法容纳您的汽车。
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${C_p = 1}$ - 车库尺寸刚好适合汽车，只能容纳您的汽车。
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${C_p = 2}$ - garage size is two times than your car and can accomodate two cars at a time.
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${C_p = 3}$ - garage size is three times than your car and can accomodate three cars at a time.
Process Performance
Process performance works to check the conformance of the sample generated using the process. It is expressed as a process performance index ${P_p}$. It checks whether it is meeting customer requirement or not. It varies from Process Capability in the fact that Process Performance is applicable to a particular batch of material. Sampling method may need to be quite substancial to support of the variation in the batch. Process Performance is only to be used when a process control cannot be evaluated. ${P_p}$ is governed by following formula: