SPC Runs Rules

For a list of SPC Runs Rules, see the NIST entry on the Western Electric Company (WECO) Rules :

When exploring SPC data, Runs Rules should be checked.  Violations of runs rules (and other assumptions) should be explored, with an accompanying statement in the report.

Be aware that checking for all Runs Rules violations may inflate Type 1 error.

See also Model Assumptions

It would be best if the cause for the runs rules violation could be found and addressed.  Recognizing that this is not always possible, the following is one possible statement for inclusion in a report:
  • "The Run Rules violations present in the X-bar charts were explored, although no definitive causes could be determined.  Possible explanations may include part handling variation during sample collection and/or slight inconsistencies in molding-to-measurement times."

Runs Rules are commonly associated with variables control charting, such as X-bar and R charts.