*Specialized Control Charts* |

## Preliminary Examination of Variation

As a preliminary step in the analysis, the data are sorted
by lane and visually screened for outliers (test values
greater than 130) with box plots created as follows:

proc sort data=film;
by lane;

symbol v=dot c=yellow;
title 'Outlier Analysis';
proc shewhart data=film;
boxchart testval*lane / boxstyle = schematicid
idsymbol = dot
cframe = vigb
cboxfill = vlib
cboxes = yellow
vref = 130
vreflab = 'Outlier Cutoff'
hoffset = 5
nolegend
stddevs
nolimits ;
id sample;
run;

Specifying BOXSTYLE=SCHEMATICID requests schematic box
plots with outliers identified by the value of the ID
variable SAMPLE. The STDDEVS option specifies that the
estimate of the process standard deviation is to be
based on subgroup standard deviations.
Although this estimate is not needed here
because control limits are not displayed, it is
recommended that you specify the STDDEVS option whenever you are
working with subgroup sample sizes greater than ten.
The NOLEGEND and NOLIMITS options suppress the subgroup sample
size legend and control limits for lane means that are
displayed by default.
The display is shown in Figure 49.8.

**Figure 49.8:** Outlier Analysis for the Data Set FILM

Figure 49.9 shows similarly created box plots
for the data in FILM after the outliers have been
removed.^{*}

data film;
set film;
if testval < 130;

symbol v=dot c=yellow;
title 'Variation Within Lane';
proc shewhart data=film;
boxchart testval*lane / boxstyle = schematicid
boxwidth = 5
idsymbol = dot
cframe = vigb
cboxfill = vlib
cboxes = yellow
hoffset = 5
nolegend
stddevs
nolimits ;
id sample;
run;

**Figure 49.9:** The Data Set FILM Without Outliers

Since you have no additional information about the process,
you may want to create a conventional and *R*
chart for the test values grouped by the variable SAMPLE.
This is a straightforward application of the XRCHART
statement in the SHEWHART procedure.

proc sort data=film;
by sample;

symbol v=dot c=yellow;
title 'Shewhart Chart for Means and Ranges';
proc shewhart data=film;
xrchart testval*sample /
split = '/'
npanelpos = 60
limitn = 4
coutfill = red
cframe = vigb
cinfill = vlib
cconnect = yellow
nolegend
alln;
label testval='Average Test Value/Range';
run;

The and *R* chart is displayed in
Figure 49.10. Ordinarily, the out-of-control
points in Figure 49.10 would indicate that
the process is not in statistical control. In this
situation, however, the process is known to be quite
stable, and the data have been screened for outliers.
Thus, the control limits seem to be inappropriate for the data.

**Figure 49.10:** Conventional and *R* Chart

Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.