The dashed horizontal lines in the following illustrations represent distances of 1σ and 2σ from the center line.
Apply certain tests based on your knowledge of the process. If it is likely that your data might contain particular patterns, you can look for them by choosing the appropriate test. Adding more tests makes the chart more sensitive, but may also increase the chance of getting a false signal. When you use several tests together, the chance of obtaining a signal for lack-of-control increases.
Experts recommend that you use both Test 1 and Test 2 when you create a G chart because the G chart may be slow to detect small to moderate decreases in the average number of days or number of opportunities between events.
To detect high rates of an event on a G chart, Minitab also includes the Benneyan test. The minimum data value for a G chart is 0. In most cases, the lower control limit for a G chart is also 0. Thus, in most cases, no points on a G chart can be below the lower control limit. The Benneyan test fails if there are too many consecutive points that equal 0.
Experts recommend 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.
Tests 1–8 are available for most variables control charts. Note that only tests 1–4 are available for R, S, and moving range charts.
Tests 1–4 are available for the attribute control charts.
On time-weighted control charts, Minitab only performs a test for points that go beyond the control limits. The other seven tests assume that the points are independent. Because the plotted points on time-weighted charts combine information from previous subgroups, the points are not independent.
When you create most control charts, you can select the tests to perform and change the parameters for each test. For example, suppose that you create an Xbar chart and you want to perform tests 1, 2, and 7. In addition, you want to draw control limits at 2.5σ instead of 3σ so that it is easier for points to fail test 1.
You can change the default tests and test parameters for future sessions of Minitab. For example, suppose you want to perform all test for special causes whenever you create a control chart. In addition, you want to draw the control limits for all control charts at 2.5σ instead of 3σ.
This procedure will not undo any changes that you have made in the Tests tab for specific graphs in existing projects. For example, if you previously modified the parameters for an Xbar chart in the current project, Minitab will remember your settings for that chart. The new defaults will not be applied to Xbar charts in the current project.