Step 3: Displaying the Trend Chart

The third step is to create a trend chart with the SHEWHART procedure, as follows:

symbol v = none w=5;
title 'Trend Chart for Diameter';
proc shewhart history=regdata;
   xchart Diameter*hour /
      wtrend   = 1
      trendvar = fitted
      split    = '/'
      stddevs
      nolegend;
   label Diameterx = 'Residual Mean/Fitted Mean';
   label hour     = 'Hour';
run;

The chart is shown in Figure 15.169. The values of FITTED are plotted in the lower half of the trend chart. The upper half of the trend chart is an chart for the residual means (DiameterX FITTED). The CNEEDLES= option specifies that the residuals are to be represented by vertical bars as deviations from the central line. The chart in Figure 15.169 shows that, after accounting for the trend, the mean level of the process is in control.

Figure 15.169 Trend Chart for Diameter Data