UCHART Statement: SHEWHART Procedure

DATA= Data Set

You can read defect counts for subgroup samples from a DATA= data set specified in the PROC SHEWHART statement. Each process specified in the UCHART statement must be a SAS variable in the data set. This variable provides the defect count (number of nonconformities) for subgroup samples indexed by the subgroup-variable. The subgroup-variable, specified in the UCHART 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. The data set should contain one observation per subgroup. When you use a DATA= data set with the UCHART statement, the SUBGROUPN= option (which specifies the number of inspection units per subgroup) is required. 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 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 u Charts from Defect Count 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;
   uchart Defects*Lot / subgroupn = 10;
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 _LCLU_, _U_, and _UCLU_, which specify the control limits

• the variable _U_, which is used to calculate the control limits (see Control Limits)

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' and 'STANDARD'.

• 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 procedure or to read output data sets created with SAS summarization procedures.

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

• subgroup-variable

• subgroup number of nonconformities per unit variable for each process

• subgroup sample size variable (number of units per subgroup) for each process

The names of the variables containing the number of nonconformities per unit and subgroup sample sizes must be the process name concatenated with the special suffix characters and , respectively. For example, consider the following statements:

proc shewhart history=Summary;
   uchart (Flaws nDefects)*Lot;
run;

The data set Summary must include the variables Lot, FlawsU, FlawsN, nDefectsU, and nDefectsN.

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) with the READPHASES= option (see Displaying Stratification in Phases for an example).

For an example of a HISTORY= data set, see Creating u Charts from Nonconformities per Unit.

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 create your own TABLE= data set. Because the SHEWHART procedure simply displays the information read from 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 UCHART 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 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.