Use the tests for special causes to determine which observations you may need to investigate and to identify the specific patterns and trends in your data. Minitab provides four different tests for special causes. Select additional tests based on company or industry standards.
In the drop-down list, specify whether to perform some, all, or no tests for special causes.
- One point more than 3 sigmas 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.
- Nine points in a row on same side of center line
- Test 2 identifies shifts in the process variation. 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.
- Six points in a row, all increasing or all decreasing
- Test 3 detects trends. This test looks for long series of consecutive points that consistently increase in value or decrease in value.
- Fourteen 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.