Process Capability - Statistical Process Control

The controls charts described so far are concerned with maintaining the stability of a process, but they offer no comment on the capability of the process in relation to the design requirements. To do this, a process capability study may be conducted to compare the common cause process variability to the tolerance band stipulated by the product designer. Typically, such a tolerance band is denoted by an upper specification limit (USL) and a lower specification limit (LSL). 

The uses of process capability studies include: 

1.  Predicting whether or not design specifications can be met 

2.  Evaluating equipment being considered for purchase 

3.  Allocating products to processes 

4.  Analysing the interrelationships of sequential processes 

5.  Adjusting process settings. 

Two indices of process capability are commonly used, Cp and C_pk.