SPC Procedure

includes all subgroups in the analysis, regardless of whether the subgroup sample size equals the nominal control limit sample size n that you specify in the LIMITN= option or the variable _LIMITN_ in the LIMITS= data table. Use the ALLN option in conjunction with the LIMITN= option or the variable _LIMITN_.

requests constant lower and upper control limits for all subgroups in a moving average chart.

In a uniformly weighted moving average chart, asymptotic limits have the following values:

Here w is the span of the moving average, and n is the nominal sample size associated with the control limits. When you do not specify this option, the control limits are computed using the exact formulas in Table 25.

In an exponentially weighted moving average chart, asymptotic limits have the following values:

Here r is the weight parameter , and n is the nominal sample size associated with the control limits. When you do not specify the ASYMPTOTIC option, the control limits are computed using the exact formulas in Table 26.

Use the ASYMPTOTIC option only if all the subgroup sample sizes are the same or if you specify a nominal subgroup sample size by using the LIMITN= option or by including the _LIMITN_ column in a LIMITS= data table.

This option is available only in the EWMACHART and MACHART statements.

requests capability index confidence limits that are based on subgroup summary data and are calculated using "effective degrees of freedom," as described by Bissell (1990). These confidence limits are approximate. When you specify the CIINDICES option, the calculated confidence limits are displayed in the "CapabilityIndices" table and are included in the OUTLIMITS= data table, if one is produced.

The CIINDICES option is ignored unless you also specify the SPECS= option. These options are available only in the chart statements that produce Shewhart charts for variables: BOXCHART, IRCHART, MCHART, MRCHART, RCHART, SCHART, XCHART, XRCHART, and XSCHART. For more information about capability indices, see the section Capability Indices.

specifies whether the control limits that are displayed in a box chart are computed for subgroup means or for subgroup medians. By default, CONTROLSTAT=MEAN. This option is available only in the BOXCHART statement.

includes a control chart in the results only when exceptions occur—specifically, when the control limits are exceeded or when any tests that you request by using the TESTS= or TESTS2= option are positive.

specifies a nominal sample size for the control limits, where n is a whole number.

If you specify LIMITN=n, the control limits are computed for the fixed value n, and they do not vary with the subgroup sample sizes. Moreover, only those subgroups whose sample size is equal to n are included in the analysis. You can specify the ALLN option in conjunction with LIMITN=n to force all subgroups to be included, regardless of subgroup sample size.

Note that control limits for EWMA and moving average charts vary according to the number of subgroup means that are included in the moving average, not only because of varying subgroup sample sizes.

If you do not specify LIMITN=n and the subgroup sample sizes are constant, the default value of n is the constant subgroup sample size.

Depending on which chart statement you use, there are restrictions on the value of n that you can specify in the LIMITN= option:

This option is not available in the IRCHART statement.

Notice the difference between the LIMITN= option and the SUBGROUPN= option that is available in the CCHART, NPCHART, PCHART, and UCHART statements. The LIMITN= option specifies a nominal sample size for the control limits, whereas the SUBGROUPN= option provides the sample sizes for the data.

Note: As an alternative to specifying the LIMITN= option, you can read the nominal control limit sample size from the variable _LIMITN_ in a LIMITS= data table.

identifies a method of estimating the process mean , which is represented by the central line on a median chart. The method that corresponds to each keyword is shown in Table 16.

By default, MEDCENTRAL=AVGMED. This option is available only in the MCHART and MRCHART statements, and in the BOXCHART statement when you specify the CONTROLSTAT=MEDIAN option.

suppresses the check for limits when you request tests for special causes. This enables tests for special causes to be applied when you use SIGMAS= option to specify control limits other than the default limits. You should not use the NO3SIGMACHECK option for standard control chart applications, because the standard tests for special causes assume limits.

This option is not available in the EWMACHART and MACHART statements.

suppresses the display of ODS output.

creates an output data table that saves the control limits and/or control limit parameters. You can use an OUTLIMITS= data table as an input LIMITS= data table in a subsequent run of the procedure. libref.data-table is a two-level name, where libref refers to the library, and data-table specifies the name of the output data table. For more information about this two-level name, see the DATA= option and the section Using SAS Viya Workbench.

For more information about OUTLIMITS= data tables, see the section OUTLIMITS= Data Table.

creates an output data table that saves the information plotted on the chart, including the subgroup variable values and their corresponding summary statistics and control limits. libref.data-table is a two-level name, where libref refers to the library, and data-table specifies the name of the output data table. For more information about this two-level name, see the DATA= option and the section Using SAS Viya Workbench.

For more information about OUTTABLE= data tables, see the section OUTTABLE= Data Table.

specifies one of five definitions that are used to calculate percentiles in the construction of box-and-whisker plots that you request in the BOXCHART statement. The index can be 1, 2, 3, 4, or 5. The five corresponding percentile definitions are discussed in the section Percentile Definitions. By default, PCTLDEF=5. This option is available only in the BOXCHART statement.

resets the value of the EWMA after each out-of-control point. Specifically, when a point exceeds the control limits, the EWMA for the next subgroup is computed as the weighted average of the subgroup mean and the overall mean. By default, the EWMA values are not reset. This option is available only in the EWMACHART statement.

specifies the width of the control limits in terms of the multiple k of the standard error of the subgroup summary statistic. The value of k must be positive. By default, k = 3, and the control limits are limits.

specifies the method of estimating the process standard deviation, , as summarized in Table 17.

Table 17: Methods of Estimating the Process Standard Deviation

Keyword	Method
MAD	Calculates a median absolute deviation estimate of
MMR	Calculates a median moving range estimate of
MVGRANGE	Estimates based on a moving range of n subgroup means
RMSDF	Calculates a root mean square estimate of
RMVLUE	Calculates a minimum variance linear unbiased estimate of based on subgroup ranges
RNOWEIGHT	Estimates as an unweighted average of unbiased subgroup estimates of based on subgroup ranges
SMVLUE	Calculates a minimum variance linear unbiased estimate of based on subgroup standard deviations
SNOWEIGHT	Estimates as an unweighted average of unbiased subgroup estimates of based on subgroup standard deviations

Table 18 lists the SMETHOD= option values that are valid for each chart statement. The default value for each statement is shown in bold.

When you use the MMR or MVGRANGE method, you can specify the number n of consecutive subgroup summary statistics from which moving ranges are computed by appending the number in parentheses to the keyword. For example, you can specify SMETHOD=MVGRANGE(4) to use moving ranges of four subgroup statistics. Values in the range are valid. By default, .

This option is not available in the following statements that produce Shewhart charts for attributes: CCHART, NPCHART, PCHART, and UCHART.

For more information about the different methods of estimating the process standard deviation, see the section Methods of Estimating the Standard Deviation.

specifies the number of terms to use in calculating the moving average for a uniformly weighted moving average chart, where w is an integer greater than 1. This option is required in an MACHART statement unless you specify span values in a column named _SPAN_ in a LIMITS= data table.

This option is available only in the MACHART statement.

specifies an input data table that contains process specification limits. libref.data-table is a two-level name, where libref refers to the library, and data-table specifies the name of the input data table. For more information about this two-level name, see the DATA= option and the section Using SAS Viya Workbench. PROC SPC calculates capability indices for processes whose specifications are given in data-table.

This option is available only in the statements that produce Shewhart charts for variables: BOXCHART, IRCHART, MCHART, MRCHART, RCHART, SCHART, XCHART, XRCHART, and XSCHART. For more information, see the sections SPECS= Data Table and Capability Indices.

specifies a variable in the input data table whose values specify the subgroup sample sizes. This option is available only in the CCHART, NPCHART, PCHART, and UCHART statements. You must specify this option in the NPCHART, PCHART, and UCHART statements. For the CCHART statement, the default subgroup sample size is 1.

specifies the length of the pattern for Test 2 that you request in the TESTS= and TESTS2= options. The run-length values that you can specify are 7, 8, 9, 11, 14, and 20. The form of the test for each run-length value is shown in the following table. By default, TEST2RUN=9. For more information, see the section Tests for Special Causes.

This option is not available in the EWMACHART and MACHART statements.

specifies the length of the pattern for Test 3 that you request in the TESTS= and TESTS2= options. Test 3 searches for a pattern of steadily increasing or decreasing values, where the length of the pattern is at least the value of run-length. The values that you can specify for run-length are 6, 7, and 8. By default, TEST3RUN=6. For more information, see the section Tests for Special Causes.

This option is not available in the EWMACHART and MACHART statements.

applies the tests for special causes that you request in the TESTS= and TESTS2= options to standardized test statistics when the subgroup sample sizes are not constant. This method was suggested by Nelson (1994). For more information, see the section Tests for Special Causes. By default, the tests are not applied to data that have varying subgroup sample sizes.

This option is not available in the EWMACHART and MACHART statements.

applies tests for special causes (which you request in the TESTS= or TESTS2= option) to overlapping patterns of points.

This option modifies the way in which the search for a subsequent pattern is done when a pattern is encountered. If you omit this option, the search begins with the first subgroup after the current pattern ends. If you specify this option, the search begins with the second subgroup in the current pattern.

The following statements request Test 3, which detects six subgroup means in a row that steadily increase or decrease:

proc spc;
   xrchart / test=3;
   xrchart / test=3 testoverlap;
run;

Suppose that the subgroup means are steadily increasing for subgroups 5, 6, 7, 8, 9, 10, and 11. The first XRCHART statement signals that Test 3 is positive at subgroup 10 but not at subgroup 11, because the search for the next pattern begins with subgroup 11. The second XRCHART statement signals that Test 3 is positive at subgroup 10 and subgroup 11, because the search for the next pattern begins with subgroup 6 and thus finds a second pattern that ends with subgroup 11. For more information, see the section Tests for Special Causes.

Caution: Specifying TESTOVERLAP affects the interpretation of the standard tests for special causes, because a particular point can contribute to more than one positive test. Typically, you should not use this option.

This option is not available in the EWMACHART and MACHART statements.

requests one or more tests for special causes, which are also known as runs tests, pattern tests, and Western Electric rules. These tests detect particular nonrandom patterns in the points that are plotted on the primary control chart. The occurrence of a pattern, referred to as a signal, suggests the presence of a special cause of variation.

Each pattern is defined in terms of Zones A, B, and C, which are constructed by dividing the interval between the control limits into six equally spaced subintervals. Zone A is the union of the subintervals immediately below the upper control limit and immediately above the lower control limit. Zone C is the union of the subintervals immediately above and below the central line. Zone B is the union of the subintervals between Zones A and C. For an illustration of test zones, see Figure 9.

Table 19 lists the standard tests that you can request by specifying this option. The tests are indexed according to the sequence used by Nelson (1984, 1985).

You can specify any combination of the eight indices by using an explicit list or an implicit list, as in the following example:

proc spc;
   xrchart / tests=1 2 3 4;
   xrchart / tests=1 to 4;
run;

The TESTS= option is available in all chart statements except the RCHART and SCHART statements. Use only tests 1, 2, 3, and 4 in the CCHART, NPCHART, PCHART, and UCHART statements. By default, the TESTS= option is not applied in any chart statement unless the control limits are limits. You can use the NO3SIGMACHECK option to request tests for special causes when you use the SIGMAS= option to specify control limits other than limits.

For more information about the TESTS= option, see the section Tests for Special Causes. Related options include TEST2RUN=, TEST3RUN=, TESTNSTD, TESTOVERLAP, and TESTS2=.

This option is not available in the EWMACHART and MACHART statements.

requests one or more tests for special causes for R charts or s charts. The syntax for the TESTS2= option is identical to the syntax for the TESTS= option. The TESTS2= option is available in the MRCHART, RCHART, SCHART, XRCHART, and XSCHART statements. For more information about the TESTS2= option, see the section Tests for Special Causes. Related options include TEST2RUN=, TEST3RUN=, TESTNSTD, TESTOVERLAP, and TESTS=.

specifies the weight r () assigned to the most recent subgroup mean in the computation of the exponentially weighted moving average in an EWMA chart. The WEIGHT= option is required in an EWMACHART statement unless you specify weight values in a column named _WEIGHT_ in a LIMITS= data table.

This option is available only in the EWMACHART statement.