BOXCHART Statement: SHEWHART Procedure

You can read raw data (process measurements) from a DATA= data set specified in the PROC SHEWHART statement. Each process specified in the BOXCHART statement must be a SAS variable in the data set. This variable provides measurements which must be grouped into subgroup samples indexed by the subgroup-variable. The subgroup-variable, specified in the BOXCHART 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 ith subgroup contains measurements, there should be consecutive observations for which the value of the subgroup-variable is the index of the ith subgroup. For example, if each subgroup contains 20 items and there are 30 subgroup samples, the DATA= data set should contain 600 observations. Other variables that can be read from a DATA= data set include

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

For an example of a DATA= data set, see Creating Box Charts from Raw Data.

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:

proc shewhart data=Info limits=Conlims;
   boxchart 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; see Table 18.7. 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:

In addition, note the following:

For an example, see Reading Preestablished Control Limits.

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, CUSUM, or MACONTROL procedures or to read output data sets created with SAS summarization procedures, such as PROC UNIVARIATE.

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

If you specify the RANGES option, the subgroup range variable must be included; otherwise, the subgroup standard deviation variable must be included.

The names of the subgroup summary statistics variables must be the process name concatenated with the following special suffix characters:

For example, consider the following statements:

proc shewhart history=summary;
   boxchart (weight Yieldstrength)*batch;
run;

The data set Summary must include the variables Batch, WeightL, Weight1, WeightM, WeightX, Weight3, WeightH, WeightS, WeightN, YieldstrengthL, Yieldstrength1, YieldstrengthM, YieldstrengthX, Yieldstrength3, YieldstrengthH, YieldstrengthS, and YieldstrengthN.

If the RANGES option were specified in the preceding BOXCHART statement, it would be necessary for Summary to include the variables WeightR and YieldstrengthR rather than WeightS and YieldstrengthS.

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

By default, the SHEWHART procedure reads all of 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) with the READPHASES= option (see Displaying Stratification in Phases for an example).

For an example of a HISTORY= data set, see Creating Box Charts from Subgroup Summary Data.

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. 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.

Table 18.10 lists the variables required in a TABLE= data set used with the BOXCHART statement:

Note that if you specify CONTROLSTAT= MEDIAN, the variables _LCLM_, _SUBMED_, and _UCLM_ are required; otherwise, the variables _LCLX_, _SUBX_, and _UCLX_ are required.

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

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

You can read summary statistics, control limits, and outlier values from a BOX= data set specified in the PROC SHEWHART statement. This enables you to reuse an OUTBOX= data set created in a previous run of the SHEWHART procedure to display a box chart.

A BOX= data set must contain the following variables:

Each observation in a BOX= data set records the value of a single feature of one subgroup’s box-and-whisker plot, such as its mean. The _TYPE_ variable identifies the feature whose value is recorded in a given observation. Table 18.11 lists valid _TYPE_ variable values:

The features identified by the _TYPE_ values N, LCLM or LCLX, UCLM or UCLX, PROCMED or PROCMEAN, MIN, Q1, MEDIAN, MEAN, Q3, and MAX are required for each subgroup.

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

When you specify one of the keywords SCHEMATICID or SCHEMATICIDFAR with the BOXSTYLE= option, values of _ID_ are used as outlier labels. If _ID_ does not exist in the BOX= data set, the values of the first variable listed in the ID statement are used.