Statistical Process Control (SPC) Resource Center

Interpretting Control Charts

There are patterns or rules that help us determine whether a process has gone out-of-control.

  • Four Western Electric rules.
  • Five Automotive Industry Action Group (AIAG) rules.
  • Your company may have rules based on these rules or on the rules from other organizations.

Control Chart Rules

Both the Western Electric and AIAG rules cite the need to look for other unnatural or non-random patterns of variation when interpreting control charts.

Corrective action must be taken when one of these patterns develops.

  • Your company will determine the exact steps you must take to correct the situation.

Corrective action includes:

  • Identifying the out-of-control situation,
  • Looking at the Comments section to help determine why the process has changed, and
  • Making a correction.