Step 1: Preliminary Mean and Standard Deviation Charts

The following statements create $\bar{X}$ and s charts for the diameter data:

symbol h=3.0 pct;
title f=qcfont1 'X ' f=none 'and s Chart for Diameter';
proc shewhart data=toolwear;
   xschart Diameter*hour /
      outhistory = submeans
      nolegend ;
   label Diameter = 'Mean in mm';
   label hour     = 'Hour';
run;

The charts are shown in Figure 17.164. The subgroup standard deviations are all within their control limits, indicating the process variability is stable. However, the $\bar{X}$ chart displays a nonlinear trend that makes it difficult to decide if the process is in control. Subsequent investigation reveals that the trend is due to tool wear.

Figure 17.164: $\bar{X}$ and s Charts for TOOLWEAR Data

Note that the symbol $\bar{X}$ is displayed in the title with the special font QCFONT4, which matches the SWISS font used for the remainder of the title. See Appendix D: Special Fonts in SAS/QC Software, for a description of the fonts available for displaying $\bar{X}$ and related symbols.