Shewhart Control Charts for Variables

WECO-Nelson Warning Rules for Monitoring a Process Variable

  • The WECO zones represent the area between the UCL and LCL, which is divided into six zones, each having a width of σ. The zones are labeled C, B, A on each side of the center-line.
  • Aabel's implementation of tests are those described in Western Electric Company (WECO) (1956)* and Nelson (1984)** procedure rules.
  • Aabel allows visualizing the WECO zones and generating the WECO/Nelson warning rules for X(bar) S QC Charts, X(bar) R QC Chart, and individual measurement charts.
The WECO-Nelson Rules and Indicated Problems

Quality Control: WECO-Nelson Warning Rule 1

  • Rule 1: This warning rule will be triggered when 1 point is beyond zone A.
  • Problem indicated: Some point(s) is/are largely out-of-control.

Quality Control: WECO-Nelson Warning Rule 2

  • Rule 2: Rule 2: This warning rule will be triggered when 9 or more points in a row are on the same side of the center-line (mean).
  • Problem indicated: A probable shift in the process average; extended bias.

Quality Control: WECO-Nelson Warning Rule 3

  • Rule 3: This warning rule will be triggered when 6 or more points in a row are continually increasing or decreasing.
  • Problem indicated: The test signals a drift in the process average, which could be due to tool wear, poor maintenance, etc.

Quality Control: WECO Nelson Warning Rule 4

  • Rule 4: This warning rule will be triggered when 14 or more points in a row alternate in direction, i.e., moving up and down.
  • Problem indicated: Oscillation beyond noise; for example, monitoring samples linked from two alternating suppliers or two alternating shifts.

Quality Control: WECO-Nelson Warning Rule 5

  • Rule 5: This warning rule will be triggered when 2 out of 3 points in a row are in zone A or beyond, in same direction.
  • Problem indicated: An early indicator of a fair process shift.

Quality Control: WECO-Nelson Warning Rule 6

  • Rule 6: This warning rule will be triggered when 4 out of 5 points are in zone B or beyond, in the same direction.
  • Problem indicated: A strong tendency for samples to be slightly out-of-control.

Quality Control: WECO-Nelson Warning Rule 7

  • Rule 7: This warning rule will be triggered when 15 points in a row are all within zone C, on either side of the center-line (mean).
  • Problem indicated: Based on the current control limits, the indicated variability is smaller than expected.

Quality Control: WECO Nelson-Warning Rule 8

  • Rule 8: This warning rule will be triggered when 8 points in a row are in zone B or beyond, with none of the 8 points in zone C.
  • Problem indicated: Likely bimodal distribution of mean, e.g., monitoring samples produced by different machines and hence different sample being affected by different factor.

* Western Electric Company (1956), Statistical Quality Control Handbook, currently referred to as the AT&T Statistical Quality Control Handbook.

** Nelson, L. (1984). "The Shewhart Control Chart—Tests for Special Causes," Journal of Quality Technology, 15, 237-239.