Process capability compares the output of an in-control process to the specification limits set by the customer. 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. National Institute of Standards and Technology
There are several measure to calculate capability. Two important measures are Cp and Cpk. Cp is used when the mean of the specification limits is equal to the true mean while Cpk is used when its not.
Cp = (USL-LSL)/6σ
However, sometimes it may not be possible to get the True mean and True standard deviation. When that happens, we use sample deviation and Xbar