BOXCHART Statement: SHEWHART Procedure

Note: See Varying Width Box-and-Whisker Plots in the SAS/QC Sample Library.

The control limits in Output 17.4.1 apply to the subgroup means. This example illustrates how you can modify the chart to indicate whether the variability of the process is in control. The following statements create a box chart for Delay in which a dashed outline and a light gray fill color are used for a box-and-whisker plot if the corresponding subgroup standard deviation exceeds its limits.

First, the SHEWHART procedure is used to create an OUTTABLE= data set (Delaytab) that contains a variable (_EXLIMS_) that records which standard deviations exceed their limits.

proc shewhart data=Times2;
   xschart Delay * Day / nochart
                         outtable = Delaytab;
run;

Then, this information is used to set the line styles and fill colors as follows:

data Delaytab;
   length Boxcolor $ 8 ;
   set Delaytab;
   keep Day Boxcolor;
   if _exlims_ = 'UPPER' or _exlims_ = 'LOWER' then do;
      Boxcolor = 'Outside' ;
   end;
   else do;
      Boxcolor = 'Inside' ;
   end;
run;

data Times2;
   merge Times2 Delaytab;
   by Day;
run;

The following statements create the modified box chart:

ods graphics off;
title  'Analysis of Airline Departure Delays' ;
title2 '--- Standard Deviation Out of Control';
proc shewhart data=Times2;
   boxchart Delay * Day /
        nohlabel
        boxstyle      = schematic
        boxfill       = ( Boxcolor )
        boxwidthscale = 1
        odstitle      = title;
run;

The chart is shown in Output 17.5.1. The values of the variable Linestyle specified with the LBOXES= option determine the outline styles for the box-and-whisker plots. The values of the variable Boxcolor specified with the CBOXFILL= option determines the fill colors. For further details, see the entries for these options in Dictionary of Options: SHEWHART Procedure. The chart indicates that the large variability for March 2 should be checked.