SPC Part 2 Rules | Dassault Systèmes®

GS 2021-12-29

DELMIA Manufacturing Operations Management

In the previous post, we discussed about various types of Statistical Process Control Charts (SPCs) with examples. We saw 2 types of control charts - Variable Control Charts and Attribute Control Charts.

Rules can be set up in the Control charts to alert the Quality team of any variation in the process.

The normal SPC control limit rules display at the 3-sigma level, both high and low. In this case, a simple threshold test determines if a process is in, or out of control.

Once a process is brought under control using the simple 3-sigma level tests, quality engineers often want to increase the sensitivity of the control chart, detecting and correcting problems before the 3-sigma control limits are reached.

These rules utilize historical data and look for a non-random pattern that can signify that the process is out of control, before reaching the normal ±3 sigma limits. The most popular of these are the Western Electric Rules, also known as the WECO Rules, or WE Runtime Rules.

Rules	Description
One point outside specification limit	The alert to be triggered if at least one point on the chart is outside the specification limit
Last point outside specification limit	The alert to be triggered if the last point on the chart is outside the specification limit.
One point outside control limit	The alert to be triggered if at least one point on the chart is outside the control limit.
Last point outside control limit	The alert to be triggered if the last point on the chart is outside the control limit.
Zone A	Zone A is defined as the range in the chart from 2 Sigma and beyond. When comparing a plotted value to the ZoneLimits, the anything in that zone and beyond is valid for processing of the alert rule.
Zone B	Zone B is defined as the range in the chart from 1 Sigma and beyond. When comparing a plotted value to the ZoneLimits, anything in that zone and beyond to be valid for processing of the alert rule.
Points in a row	A given number of points in a row will be checked for whether they increase or decrease consecutively
Alternating Points	A given number of points in a row will be checked for whether they alternate (as in, they go up, down, up, down, etc.)
WECO Rules	Determines if the alert should use WECO rules or not. The following are the rules included: Any single data point falls outside the 3σ-limit from the centerline (i.e., any point that falls outside Zone A, beyond either the upper or lower control limit). Two out of three consecutive points fall beyond the 2σ-limit (in zone A or beyond), on the same side of the centerline. Four out of five consecutive points fall beyond the 1σ-limit (in zone B or beyond), on the same side of the centerline. Eight consecutive points fall on the same side of the centerline (in zone C or beyond). Six (or more) points in a row are continually increasing (or decreasing). Fourteen (or more) points in a row alternate in direction, increasing then decreasing.