Precise Detection of Small Shifts With WECO Rules 
Courtesy of SQC Online, http://www.sqconline.com/
Courtesy of SQC Online, http://www.sqconline.com/
Shewhart-type charts are efficient in detecting medium to large shifts, but are insensitive to small shifts. One way to increase the power of Shewhart-type charts is by adding supplementary stopping rules based on runs.
Benefits of WECO Rules
- Detecting short term shifts requires real-time monitoring
- Traditional control charts cannot detect short term trends
- Short term trends can avoid issues before they occur
- TQM requires utilizing WECO rules during production
The most popular stopping rules have been suggested by the Western Electric Company (WECO). These rules supplement the ordinary rule of "One point exceeds the control limits." The most popular WECO rules include the following:
- 2 of 3 consecutive points fall outside warning (2-sigma) limits but are within control (3-sigma) limits.
- 4 of 5 consecutive points fall beyond 1-sigma limits but are within control limits.
- 8 consecutive points fall on one side of the centerline (progressively increasing or progressively decreasing trend).
To detect small shifts with WECO rules, a real-time, sequential processing capability is required. As the industry’s leading solution for on-line monitoring and alerting, SwifTest system is an ideal choice for today’s semiconductor manufacturing.
