Rational subgroup

parts made under the same conditions

A small group of parts, typically 3 to 5 consecutive ones, chosen so that only short-term variation occurs within the group and all changes happen between groups. Rational subgrouping is what makes sigma within and the control limits meaningful.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Sigma within: short-term standard deviation
• X-bar R chart: control chart for subgroup means and ranges