1 In the C chart dialog box, click C Chart Options. Click the S Limits tab.
2 Do any of the following:
• To specify where control limits are drawn: under Display control limits at, enter one or more values in These multiples of the standard deviation. Each value is the number of standard deviations the lines should be drawn at, above, and below the mean.
• To set bounds on the control limits: check Lower standard deviation limit bound (and/or Upper standard deviation limit bound) and enter a value. Each value represents the number of standard deviations below and above the mean.
3 Click OK in each dialog box.
Options − Tests
... > control chart Options > Tests Charts R, S, Moving Range, P, NP, U, C
With any of the attributes control charts, you can perform the 4 tests for special causes. Each test, as shown below, 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 [10] 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.
Test 1 1 point more than 3σ from center line Test 2 9 points in a row on same side of center line
Test 3 6 points in a row, all increasing or all decreasing Test 4 14 points in a row, alternating up and down
Dialog box items
Perform all tests for special causes: Choose to perform all tests.
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.
1 point > 3 standard deviations from center line 9 points in a row on same side of center line 6 points in a row, all increasing or all decreasing 14 points in a row, alternating up and down
Perform no tests: Choose to perform no tests for special causes.
To perform the tests for special causes
1 Choose Stat > Control Charts > control chart type > control chart > control chart Options > Tests.
2 Do one of the following:
• To do all tests, choose Perform all tests for special causes.
• To select certain tests, choose Perform the following tests for special causes and check the tests to perform.
• To perform no tests, choose Perform no tests.
3 Click OK.
Options − Stages
... > control chart Options > Stages Charts All charts except Z-MR
You can display stages in your process by drawing a "historical chart" − a control chart in which the control limits and center line are estimated independently for different groups in your data. Historical charts are particularly useful for comparing data before and after a process improvement.
Note With the following charts, you must have at least one subgroup with two or more observations: R, S, X-bar and R, and X-bar and S.
Dialog box items
Define stages (historical groups) with this variable: Enter the column that contains the stage indicators.
When to start a new stage
With each new value: Choose to start a new stage each time the value in the column changes.
With the first occurrence of these values: Choose to start a new stage at the first occurrence of a certain value, then enter the values. Enclose date/time or text entries in double quotes. You can enter the same value more than once;
Minitab treats each repeated value as a separate occurrence.
To display a historical chart
To define stages in your process, you must set up a column of grouping indicators. The indicators can be numbers, dates, or text. When executing the command, you can tell Minitab to start a new stage in one of two ways:
• Each time the value in the column changes
• At the first occurrence of one or more values
The column must be the same length as the data column (or columns, when subgroups are across rows).
1 In the control chart dialog box, click control chart Options.
2 Click the Stages tab.
3
In Define stages (historical groups) with this variable, enter the column which contains the stage indicators.
4
Under When to start a new value, do one of the following:
• To start a new stage each time the value in the column changes, choose With each new value.
• To start a new stage at the first occurrence of a certain value, choose With the first occurrence of these values.
Enter the values. Enclose date/time or text entries in double quotes. You can enter the same value more than once; Minitab treats each repeated value as a separate occurrence.
5
If you like, use any dialog box options, then click OK.
Options − Display
... > control chart Options > Display Charts All charts except Z-MR
Use to display the control chart by stage or by number of plotted points and to display the test results in the Session window.
Subgroups to display
Display all subgroups: Choose to display all subgroups.
Display last __ subgroups: Choose to display a specific number of subgroups, and enter the number of subgroups.
Split chart into series of segments for display purposes Do not split: Choose not to split the chart.
Each segment contains __ subgroups: Choose to split the control chart into subgroups of a specified number of points, and enter the number of subgroups for each segment.
Each segment corresponds to a stage (if chart has stages): Choose to split the control chart by stage or stage when you have specified stages in control chart Options > Stage.
Test results
Display test results in Session window: Check to display the test results in the Session window.
To display subgroups
1 In the control chart dialog box, click control chart Options. Click the Display tab.
2 Do one of the following:
• To display all subgroups in the control chart, under Subgroups to display, choose Display all subgroups.
• To display a specific number of the last subgroups, under Subgroups to display, choose Display last __
subgroups and enter the number of subgroups to display.
3 Click OK.
To split a control chart
1 In the control chart dialog box, click control chart Options. Click the Display tab.
2 Do one of the following:
• To split the chart into a specific number of subgroups, choose Each segment contains __ subgroups, and enter the number of subgroups for each segment.
• To split the chart by stage, choose Each segment corresponds to a stage (if chart has stages).
3 Click OK.