Select tests for special causes for Acceptance Xbar-S Chart

Minitab provides tests for special causes for the Acceptance Xbar chart and the S chart. By default, Minitab uses only the test for points outside of the Acceptance Xbar chart control limits and points on the S chart that are K standard deviations from the center line. Select additional tests based on company or industry standards. Use the tests to determine which observations to investigate, and to identify the specific patterns and trends in your data.

In the drop-down list, specify whether to perform some, all, or no tests for special causes. You can make each test more or less sensitive by changing the value of K.

1 point outside of acceptance chart control limits

Identify subgroup means that are outside the Acceptance Xbar chart control limits.

1 point > K standard deviations from center line (S chart)

Identify subgroups with standard deviations that are unusual compared to other subgroups.

K points in a row on same side of center line (S chart)

Identify shifts in the process variation.

K points in a row, all increasing or all decreasing (S chart)

Detect trends in the process variation. This test looks for a long series of consecutive points that consistently increase in value or decrease in value.

K points in a row, alternating up and down (S chart)

Detect systematic variation. You want the pattern of variation in a process to be random, but a point that fails this test might indicate that the pattern of variation is predictable.