Minitab provides four tests for special causes. By default, Minitab uses only Test 1. 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.
To change the default settings for future sessions of Minitab, choose
- 1 point > K standard deviations from center line
- Test 1 identifies subgroups that are unusual compared to other subgroups. Test 1 is universally recognized as necessary for detecting out-of-control situations. If small shifts in the process are of interest, you can use Test 2 to supplement Test 1 in order to create a control chart that has greater sensitivity.
- K points in a row on same side of center line
- Test 2 identifies shifts in the proportion of defectives for the process. If small shifts in the process are of interest, you can use Test 2 to supplement Test 1 in order to create a control chart that has greater sensitivity.
- K points in a row, all increasing or all decreasing
- Test 3 detects trends. 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
- Test 4 detects systematic variation. You want the pattern of variation in a process to be random, but a point that fails Test 4 might indicate that the pattern of variation is predictable.