Options - Tests
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Charts  P, Laney P', NP, U, Laney U', C

With any of the attributes control charts, you can perform the 4 tests for special causes. Each test, as shown in Four Tests for Special Causes detects a specific pattern in the plotted data. The occurrence of a pattern suggests a special cause for the variation, one that should be investigated. See [1] and [12] for guidance on using these tests.

When a point fails a test, Minitab marks it with the test number on the plot. If a point fails more than one test, Minitab marks it by the lowest numbered test.

You can choose to change the color of a control chart when the last row of data causes a new test failure for any point. See Tools > Options > Control Charts and Quality Tools > Other.

Set your preferences for the sensitivity of tests with Tools > Options > Control Charts and Quality Tools > Define Tests.

The test results are displayed in the Session window. If the data change and the control chart is live-linked, the test results are not updated in the Session window. See Updating Graphs for more information.

Minitab will perform tests when sample sizes are unequal.

Dialog box items

Tests For Special Causes (default definitions):

Perform all tests for special causes: Choose to perform all four tests shown below.

Perform the following tests for special causes: Choose to specify a subset of the four tests, then check any combination of the tests listed below. You may customize particular tests by changing the values of K.

 

K

1 point more than K standard deviations from center line

K determines where the control limits are drawn on the chart.

3

K points in a row on same side of center line

9

K points in a row, all increasing or all decreasing

6

K points in a row, alternating up and down

14

Perform no tests: Choose to perform no tests for special causes.