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