III. MARCO METODOLÓGICO
3.5 Técnicas e Instrumentos de Recolección de Datos
1 In the control chart data source dialog box, click control chart Options.
2 In Proportion, enter the historical proportions as constants.
3 Click OK in each dialog box.
You can enter historical data in several ways:
• A single value, which applies to all variables and stages.
• A list of values, one for each variable. All stages for each variable use the same value.
• A list of values, one for each stage within each variable. Each stage for each variable uses a different value.
Options − Estimate
... > control chart Options > Estimate Charts P, NP
You can omit or use certain subgroups to estimate p. For example, if some subgroups have erratic data due to assignable causes that you have corrected, you can prevent these subgroups from influencing the estimation of p. You can also force the control limits to be constant when you have unequal sample sizes.
Caution When you include or exclude rows using control chart > Data Options > Subset and also omit or use samples to estimate parameters, the omitted or used samples apply to the entire data set, not just the subsetted data.
For example, you exclude subgroup 1 from your analysis (using control chart > Data Options > Subset), then decide to omit subgroup 2 from the parameter estimation (using control chart Options > Estimate). Because Minitab considers an omitted sample (subgroup 1) as part of the entire data set, you must enter subgroup 2 as the subgroup to omit.
Dialog box items
Omit the following samples when estimating parameters (eg, 3 12:15): Choose to specify subgroups of the data to omit from the estimation of p, then type individual subgroup numbers or ranges of subgroups.
Use the following samples when estimating parameters (eg, 3 12:15): Choose to specify subgroups of data to compute p, then type individual subgroup numbers or a range of subgroups.
To omit or use subgroups for the estimates of parameters
By default, Minitab estimates the process parameters from all the data. But you may want to use or omit certain data if they show abnormal behavior.
1
In the control chart dialog box, click control chart Options. Click the Estimate tab.
2 Do one of the following:
• Choose Omit the following samples when estimating parameters and enter the subgroups or observation numbers you want to omit from the calculations.
• Choose Use the following samples when estimating parameters and enter the subgroups or observation numbers you want to use for the calculations.
Note Minitab assumes the values you enter are subgroup numbers, except with the Individuals, Moving Range, and I-MR charts. With these charts, the values are interpreted as observation numbers.
3
Click OK.
S Limits
... > control chart Options > S Limits
Charts X-bar, R, S, P, NP, U, Moving Average, EWMA
Allows you to draw control limits above and below the mean at the multiples of any standard deviation. You can set bounds on the upper and/or lower control limits. When the calculated upper control limit is greater than the upper bound, a horizontal line labeled UB is drawn at the upper bound instead. Similarly, if the calculated lower control limit is less than the lower bound, a horizontal line labeled LB is drawn at the lower bound instead. You can also force the control limits to be constant when sample sizes are unequal.
For an example, see Example of an X-bar chart with tests and customized control limits.
Tip You can also modify the control limits using Minitab's graph editing features.
Dialog box items Display control limits at
These multiples of the standard deviation: Enter one or more values. The values entered are the number of standard deviations above and below the center line. If you specify more than one value, a pair of control limits is drawn for each value in the list.
Place bounds on control limits
Lower standard deviation limit bound: Check to set a lower bound for the control limits. If the calculated lower control limit is less than the lower bound, a horizontal line labeled LB will be drawn at the lower bound instead.
Upper standard deviation limit bound: Check to set an upper bound for the control limits. If the calculated upper control limit is greater than the upper bound, a horizontal line labeled UB will be drawn at the upper bound instead.
Using actual sizes of the subgroups: Choose to use the subgroups defined in the main dialog box to estimate σ.
Assuming all subgroups have size: Choose to estimate the control limits using a specified subgroup size, then type the subgroup size. When subgroup sizes are not equal, each control limit is not a single straight line, but varies with the subgroup size. If the sizes do not vary much, you may want to force the control limits to be constant by entering a sample size in this text box. Only the control limits are affected; the plotted data are not changed.
To customize the control limits
1 In the control chart dialog box, click control 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.
To force control limits and center line to be constant
1 In the control chart dialog box, click control chart Options. Click the S Limits tab.
2 Under When subgroup sizes are unequal, calculate control limits, choose Assuming all subgroups have size, then enter a value. For example, enter a value of 6 to calculate the control limits and center line as if all subgroup sizes were 6.
3 Click OK.
Note You should force the control limits and center line to be constant only when the difference in size between the largest and smallest subgroups is no more than 25%. For example, suppose the largest subgroup is size 125 and the smallest is 100. You can use this method because the size difference is 25% (125 / 100 = 25%).
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.
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.
Options − Storage
Stat > Control Charts > Attributes Charts > P > P Chart Options > Storage Stat > Control Charts > Attributes Charts > NP > NP Chart Options > Storage Use to store your choice of statistics in the worksheet.
Dialog box items
Store these estimates for each chart
Proportions: Check to store the proportions, one row for each historical stage.
Store these values for each point
Point plotted: Check to store the plotted points, one row for each plotted point.
Center line value: Check to store the center line value, one row for each plotted point.
Control limit values: Check to store the control limit values, one row for each plotted point. Minitab stores one column for the lower control limits and one column for the upper control limits.
Stage: Check to store the stage, one row for each plotted point.
Subgroup size: Check to store the subgroup sizes, one row for each plotted point.
Test results: Check to store the results of any test performed, one row for each plotted point. Minitab assigns each plotted point one of three values: 1 (beyond upper control limit), − 1 (beyond lower control limit), 0 (in control).
To store statistics in the worksheet
1 Choose Stat > Control Charts > control chart type > control chart > control chart Options > Storage.
2 Check the statistics to store in the worksheet, then click OK.