MRCHART Statement: SHEWHART Procedure

DATA= Data Set

You can read raw data (process measurements) from a DATA= data set specified in the PROC SHEWHART statement. Each process specified in the MRCHART statement must be a SAS variable in the DATA= data set. This variable provides measurements that must be grouped into subgroup samples indexed by the values of the subgroup-variable. The subgroup-variable, which is specified in the MRCHART statement, must also be a SAS variable in the DATA= data set. Each observation in a DATA= data set must contain a value for each process and a value for the subgroup-variable. If the th subgroup contains items, there should be consecutive observations for which the value of the subgroup-variable is the index of the th subgroup. For example, if each subgroup contains five items and there are 30 subgroup samples, the DATA= data set should contain 150 observations.

Other variables that can be read from a DATA= data set include

• _PHASE_ (if the READPHASES= option is specified)

• block-variables

• symbol-variable

• BY variables

• ID variables

By default, the SHEWHART procedure reads all of the observations in a DATA= data set. However, if the DATA= data set includes the variable _PHASE_, you can read selected groups of observations (referred to as phases) by specifying the READPHASES= option (for an example, see Displaying Stratification in Phases).

For an example of a DATA= data set, see Creating Charts for Medians and Ranges from Raw Data.

LIMITS= Data Set

You can read preestablished control limits (or parameters from which the control limits can be calculated) from a LIMITS= data set specified in the PROC SHEWHART statement. For example, the following statements read control limit information from the data set Conlims:¹

proc shewhart data=Info limits=Conlims;
   mrchart Weight*Batch;
run;

The LIMITS= data set can be an OUTLIMITS= data set that was created in a previous run of the SHEWHART procedure. Such data sets always contain the variables required for a LIMITS= data set. The LIMITS= data set can also be created directly using a DATA step. When you create a LIMITS= data set, you must provide one of the following:

• the variables _LCLM_, _MEAN_, _UCLM_, _LCLR_, _R_, and _UCLR_, which specify the control limits directly

• the variables _MEAN_ and _STDDEV_ , which are used to calculate the control limits according to the equations in Table 15.32

In addition, note the following:

• The variables _VAR_ and _SUBGRP_ are required. These must be character variables whose lengths are no greater than 32.

• The variable _INDEX_ is required if you specify the READINDEX= option; this must be a character variable whose length is no greater than 48.

• The variables _LIMITN_, _SIGMAS_ (or _ALPHA_), and _TYPE_ are optional, but they are recommended to maintain a complete set of control limit information. The variable _TYPE_ must be a character variable of length 8; valid values are 'ESTIMATE', 'STANDARD', 'STDMU', and 'STDSIGMA'.

• BY variables are required if specified with a BY statement.

For an example, see Reading Preestablished Control Limits.

HISTORY= Data Set

You can read subgroup summary statistics from a HISTORY= data set specified in the PROC SHEWHART statement. This enables you to reuse OUTHISTORY= data sets that have been created in previous runs of the SHEWHART procedures or to read output data sets created with SAS summarization procedures, such as PROC UNIVARIATE.

A HISTORY= data set used with the MRCHART statement must contain the following variables:

• the subgroup-variable

• a subgroup mean variable for each process

• a subgroup median variable for each process

• a subgroup range variable for each process

• a subgroup sample size variable for each process

The names of the subgroup mean, subgroup median, subgroup range, and subgroup sample size variables must be the process name concatenated with the special suffix characters , , , and , respectively. You must provide the subgroup mean variable only if you specify the MEDCENTRAL=AVGMEAN option.

For example, consider the following statements:

proc shewhart history=Summary;
   mrchart (Weight Yieldstrength)*Batch / medcentral=avgmean;
run;

The data set Summary must include the variables Batch, WeightX, WeightM, WeightR, WeightN, YieldstrengthX, YieldstrengthM, YieldstrengthR, and YieldstrengthN.

Note that if you specify a process name that contains 32 characters, the names of the summary variables must be formed from the first 16 characters and the last 15 characters of the process name, suffixed with the appropriate character.

Other variables that can be read from a HISTORY= data set include

• _PHASE_ (if the READPHASES= option is specified)

• block-variables

• symbol-variable

• BY variables

• ID variables

By default, the SHEWHART procedure reads all the observations in a HISTORY= data set. However, if the data set includes the variable _PHASE_, you can read selected groups of observations (referred to as phases) by specifying the READPHASES= option (see Displaying Stratification in Phases for an example).

For an example of a HISTORY= data set, see Creating Charts for Medians and Ranges from Summary Data.

TABLE= Data Set

You can read summary statistics and control limits from a TABLE= data set specified in the PROC SHEWHART statement. This enables you to reuse an OUTTABLE= data set created in a previous run of the SHEWHART procedure or to read data sets created by other SAS procedures. Because the SHEWHART procedure simply displays the information in a TABLE= data set, you can use TABLE= data sets to create specialized control charts. Examples are provided in Specialized Control Charts: SHEWHART Procedure.

The following table lists the variables required in a TABLE= data set used with the MRCHART statement:

Other variables that can be read from a TABLE= data set include

• block-variables

• symbol-variable

• BY variables

• ID variables

• _PHASE_ (if the READPHASES= option is specified). This variable must be a character variable whose length is no greater than 48.

• _TESTS_ (if the TESTS= option is specified). This variable is used to flag tests for special causes for subgroup medians and must be a character variable of length 8.

• _TESTS2_ (if the TESTS2= option is specified). This variable is used to flag tests for special causes for subgroup ranges and must be a character variable of length 8.

• _VAR_. This variable is required if more than one process is specified or if the data set contains information for more than one process. This variable must be a character variable whose length is no greater than 32.

For an example of a TABLE= data set, see Saving Control Limits.