UCHART Statement: SHEWHART Procedure

Syntax: UCHART Statement

The basic syntax for the UCHART statement is as follows:

  • UCHART process $*$ subgroup-variable;

The general form of this syntax is as follows:

  • UCHART processes $*$ subgroup-variable <(block-variables)><=symbol-variable | ='character'> / <options >;

You can use any number of UCHART statements in the SHEWHART procedure. The components of the UCHART statement are described as follows.

process
processes

identify one or more processes to be analyzed. The specification of process depends on the input data set specified in the PROC SHEWHART statement.

  • If numbers of nonconformities per subgroup are read from a DATA= data set, process must be the name of the variable containing the numbers of nonconformities. For an example, see Creating u Charts from Defect Count Data.

  • If numbers of nonconformities per unit and numbers of inspection units per subgroup are read from a HISTORY= data set, process must be the common prefix of the appropriate variables in the HISTORY= data set. For an example, see Creating u Charts from Nonconformities per Unit.

  • If numbers of nonconformities per item, numbers of inspection units per subgroup, and control limits are read from a TABLE= data set, process must be the value of the variable _VAR_ in the TABLE= data set. For an example, see Saving Control Limits.

A process is required. If you specify more than one process, enclose the list in parentheses. For example, the following statements request distinct u charts for Defects and Flaws: 

proc shewhart data=Measures;
   uchart (Defects Flaws)*Sample / subgroupn=50;
run;

Note that when data are read from a DATA= data set with the UCHART statement, the SUBGROUPN= option (which specifies the number of inspection units per subgroup) is required.

subgroup-variable

is the variable that identifies subgroups in the data. The subgroup-variable is required. In the preceding UCHART statement, Sample is the subgroup variable. For details, see Subgroup Variables.

block-variables

are optional variables that group the data into blocks of consecutive subgroups. The blocks are labeled in a legend, and each block-variable provides one level of labels in the legend. See Displaying Stratification in Blocks of Observations for an example.

symbol-variable

is an optional variable whose levels (unique values) determine the symbol marker or character used to plot the number of nonconformities per unit.

  • If you produce a line printer chart, an 'A' is displayed for the points corresponding to the first level of the symbol-variable, a 'B' is displayed for the points corresponding to the second level, and so on.

  • If you produce traditional graphics, distinct symbol markers are displayed for points corresponding to the various levels of the symbol-variable. You can specify the symbol markers with SYMBOLn statements. See Displaying Stratification in Levels of a Classification Variable for an example.

character

specifies a plotting character for line printer charts. For example, the following statements create a u chart using an asterisk (*) to plot the points: 

proc shewhart data=Values lineprinter;
   uchart Defects*Sample='*' / subgroupn=100;
run;
options

enhance the appearance of the chart, request additional analyses, save results in data sets, and so on. The section Summary of Options, which follows, lists all options by function. Dictionary of Options: SHEWHART Procedure describes each option in detail.