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The Process Capability Index is a statistical measure that quantifies a process’s ability to meet specifications. It outputs a numerical value indicating the level of process performance in relation to specification limits.
Cpk is a process capability index that measures how close a process is running to its specification limits [(Upper specification Limit(USL) and Lower Specification Limit (LSL)], relative to the natural variability of the process.
Cpk is a standard index to state the capability of one process, the higher the Cpk value the better the process is. For instance, Machine 1 has a Cpk of 1.7 and machine 2 has a Cpk of 1.1. From the Cpk value, one can derive that Machine 1 is better than 2.
Process Capability Index shows how closely a process is able to produce the output to its overall specifications. It decides how consistent we are to our average performance.The calculation is done by separating the formula into two halves.
Process capability indices are constructed to express more desirable capability with increasingly higher values. Values near or below zero indicate processes operating off target ( μ ^ {\displaystyle {\hat {\mu }}} far from T) or with high variation.
With the process capability index calculator, you can quickly determine whether the products of your process meet specification limits (target values, usually set by customers, in which products or services are relative to customer requirements).
Use the process capability formula to assess the potential capability to meet specifications, assuming the process is centered on the target value. Here’s the formula for calculating process capability.
The most common process capability index is given by Cp, which is an estimation of what the process is capable of producing if the process mean were to be centered between the specification limits, assuming that the process output is approximately normally distributed.
Calculate the process capability index, Cpk, which considers the process means deviation from the specification limits’ midpoint. Interpret the results to determine whether the process can confidently produce products or services that meet the quality requirement.
The formula for Cpk (process capability index) is: $$Cpk = \min\left(\frac{USL - \mu}{3\sigma}, \frac{\mu - LSL}{3\sigma}\right)$$ where μ is the mean or average of the process.