Western Electric Run Rules

Following are the stopping rules suggested by the Western Electric Company (WECO). These rules supplement the ordinary rule: "One point exceeds the control limits." Here are the most popular Western Electric rules:

2 of 3 consecutive points fall outside warning (2-sigma) limits, but within control (3-sigma) limits.

4 of 5 consecutive points fall beyond 1-sigma limits, but within control limits.

8 consecutive points fall on one side of the centerline. (STATISTICA uses 9 consecutive points for this rule. See Out-Of-Control Process: Runs Tests.)

In STATISTICA, the runs rules can be accessed from the Quality Control module. Click the Runs tests button on the Specs tab of the Variable Control Chart results dialog box to display the Runs Tests for Control Chart dialog box, which contains options to select which runs tests to compute.

For a description of these tests and their interpretation, refer to Quality Control Introductory Overview - Out-Of-Control Process: Runs Tests, or to AT&T (1959), Nelson (1984, 1985), Grant and Leavenworth (1980), or Shirland (1993).