Process parameters
overview     see also 

Process capability statistics are numerical measures of process capability - that is, they measure how capable a process is of meeting specifications. For a stable process, reliable estimates can be made of the various process parameters required for capability studies. The parameters used in estimating the indices differ based on the distribution used in the analysis. Use the table below as a guide:

Assumed distribution

Capability measures

Parameters estimates used

Normal (within analysis)

Potential capability

Process mean (X)

Within-subgroup standard deviation

Normal (between/within analysis)

Between/Within capability

Process mean (X)

Between/Within standard deviation

Normal (within and within/between analysis)

Overall capability

Process mean (X)

Overall standard deviation

Nonnormal

Overall capability

Process median (50th percentile)

Process spread (99.965th percentile - 0.135th percentile)

Based on the ISO method (the default) or the Minitab method.