Options - Tests
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Stat > Control Charts > Rare Event Charts > T > T chart Options > Tests

Selects a subset of the tests shown in Eight Tests for Special Causes. Each test detects a specific pattern in the data that is plotted on the chart. The occurrence of a pattern suggests a special cause for the variation, a cause that should be investigated.

For the traditional control charts for variables data, Tests 1, 5, 6, 7 and 8 are based on the normal distribution. However, for T charts, these tests are based on the Weibull distribution or the exponential distribution. For example, a point on a T chart fails Test 1 when the point is outside of the percentile range that correspond to 3 standard deviations from the mean for a normal distribution. See Methods and Formulas for more information.

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

You can set preferences for the sensitivity of these tests using Tools > Options > Control Charts and Quality Tools > 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.

See [1] and [32] for guidance on using these tests.

Tip

Minitab recommends that you use both Test 1 and Test 2 when you create a T chart because the T chart may be slow to detect small to moderate decreases in the average time between events.

Dialog box items

Tests For Special Causes (default definitions):

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

Perform selected tests for special causes: Choose to specify a subset of the eight 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

K out of K + 1 points > 2 standard deviations from center line (same side)

2

K out of K + 1 points > 1 standard deviation from center line (same side)

4

K points in a row within 1 standard deviation of center line (either side)

15

K points in a row > 1 standard deviation from center line (either side)

8

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