Tests for special causesmain topics
Tests for special causesCharts P, Laney P', NP, U, Laney U', C, G
Each of the tests for special causes, shown below, detects a specific pattern in the data plotted on the chart. The occurrence of a pattern suggests a special cause for the variation, one that should be investigated. See [1] and [32] for guidance on using these tests.
The G chart also includes the Benneyan test, which detects high rates of a rare event by counting the number of consecutive plotted points equal to the lower control limit. When a point on a G chart fails the Benneyan test, it is marked with B.
When a point fails a test, it is marked with the test number or letter on the chart. If a point fails more than one test, the number or letter of the first test in your list is the number printed on the chart.
The tests below are shown with the default values. You can change them by changing the values of K in the Tests - Options dialog box.
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Test 1 1 point more than 3s from center line |
Test 2 9 points in a row on same side of center line |
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Test 3 Six points in a row, all increasing or all decreasing |
Test 4 Fourteen points in a row, alternating up and down |
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