# Process Capability Analysis in Excel

## You Don't Have to be a Black Belt to Perform Capability Analysis

A capable process meets customer requirements 100% of the time. Customer requirements are defined using an upper specification limit (USL) and a lower specification limit (LSL). Think of these specification limits as goal posts.

Not Capable |
Capable |
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Cp < 1, Cpk <1 |
Cp >= 1, Cpk >= 1 |

Not Capable to Centered & Capable |
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**Process capability metrics** are used to determine how well the output of a stable process meets these specifications:

## Cp and Cpk |
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Cp measures how well the data would fit within the spec limits (USL, LSL) |

Cpk measures how centered the data is between the spec limits. |

Use Cp Cpk when you have a sample, not the population, and are testing the potential capability of a process to meet customer needs. |

Cp and Cpk use Sigma estimator. |

## Pp and Ppk |
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Pp measures how well the data would fit within the spec limits (USL, LSL) |

Ppk measures how centered the data is between the spec limits. |

Use Pp Ppk when you have the total population and are testing the performance of a system to meet customer needs. |

Pp, Ppk use standard deviation. |

### Cp vs Cpk Examples

## Fits But Is Not Centered |

Process improvement goal is to make changes that will move the output closer to the target without increasing variation. |

## Centered But Does Not Fit |

Process improvement goal is to reduce variation so that the process fits between the spec limits. |

### Capability Analysis Metrics Rules of Thumb

**Cp>1**Process is capable (product will fit between the customer's upper and lower specification limits if the process is centered).**Cpk>1**Process is capable and centered between the LSL and USL.- If Cp=Cpk the process is centered at the midpoint of the specification limits.
- If Cp>Cpk the process is off-center.
- Cp and Cpk should be close in value to Pp and Ppk.
- If Cp and Cpk are much greater than Pp or Ppk (
*33% greater*), your process may not be stable enough to conduct a capability analysis. Use control charts to evaluate the stability of your process.

### QI Macros Tools that Perform Process Capability Analysis

**Histogram**- calculates Cp, Cpk, Pp, Ppk and twenty other metrics using your data and spec limits.**Capability Suite**- creates six charts including histogram, control charts, probability plot, values plot and capability plot. Also calculates Cp, Cpk and Pp, Ppk.**Cp Cpk Template**- calculates Cp, Cpk and Pp, Ppk on multiple sets of data.