The POWER Procedure

Suppose you want to add reference lines to highlight power=0.8 and power=0.9 on the plot in Output 77.8.5. You can add simple reference lines by using the YOPTS= option and REF= suboption in the PLOT statement to produce Output 77.8.11, with the following statements:

proc power plotonly;
   twosamplemeans test=diff
      groupmeans   = 12 | 15 18
      stddev       = 7 9
      power        = .
      ntotal       = 100;
   plot x=n min=20 max=500
      yopts=(ref=0.8 0.9);
run;

Or you can specify CROSSREF= YES to add reference lines that intersect each curve and cross over to the other axis:

   plot x=n min=20 max=500
      yopts=(ref=0.8 0.9 crossref=yes);

The resulting plot is shown in Output 77.8.12.

You can also add reference lines for the X axis by using the XOPTS= option instead of the YOPTS= option. For example, the following PLOT statement produces Output 77.8.13, which has crossing reference lines highlighting the sample size of 100:

   plot x=n min=20 max=500
      xopts=(ref=100 crossref=yes);

Note that the values that label the reference lines at the X axis in Output 77.8.12 and at the Y axis in Output 77.8.13 are linearly interpolated from two neighboring points on the curves. Thus they might not exactly match corresponding values that are computed directly from the methods in the section Computational Methods and Formulas—that is, computed by PROC POWER in the absence of a PLOT statement. The two ways of computing these values generally differ by a negligible amount.