# What is capability statement

## What is Process Capability?

Process capability compares the output of an *in-control* process to the specification limits by using *capability indices*. The comparison is made by forming the ratio of the spread between the process specifications (the specification "width") to the spread of the process values, as measured by 6 process standard deviation units (the process "width").

**Process Capability Indices**

*A process capability index uses both the process variability and the process specifications to determine whether the process is "capable"*

We are often required to compare the output of a stable process with the process specifications and make a statement about how well the process meets specification. To do this we compare the natural variability of a stable process with the process specification limits.

A process where almost all the measurements fall inside the specification limits is a __capable__ process.

This can be represented pictorially by the plot below:

There are several statistics that can be used to measure the capability of a process: \(C_p\), \(C_\), and \(C_\).Most capability indices estimates are valid only if the sample size used is "large enough". Large enough is generally thought to be about 50 independent data values.

The \(C_p\), \(C_\), and \(C_\) statistics assume that the population of data values is normally distributed. Assuming a two-sided specification, if \(\mu\) and \(\sigma\) are the mean and standard deviation, respectively, of the normal data and \(\mbox*Definitions of various process capability indices*