Nelson rules

tests for special causes on control charts

Eight run rules published by Lloyd Nelson (1984) that detect non-random patterns: a point beyond 3σ, nine points on one side of the centre line, six points steadily increasing or decreasing, fourteen points alternating, two of three beyond 2σ, four of five beyond 1σ, fifteen points within 1σ and eight points outside 1σ on both sides.

Using more rules detects shifts earlier but increases false alarms; many companies use rules 1 to 4.

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

• Control limits: voice of the process
• Special cause: assignable cause of variation