Introduction to Power and Sample Size Analysis: Customized Power Formulas (DATA Step)

Customized Power Formulas (DATA Step)

If you want to perform a power compuation for an analysis that is not currently supported directly in SAS/STAT tools, and you have a power formula, then you can program the formula in the DATA step.

For purposes of illustration, here is the power formula in the section Computing Power and Sample Size implemented in the DATA step to compute power for the t test example:

data tpow;
   do meandiff = 5, 6;
      do stddev = 12, 18;
         do alpha = 0.05, 0.1;
            do ntotal = 100, 200;
               ncp = ntotal * 0.5 * 0.5 * meandiff**2 / stddev**2;
               critval = finv(1-alpha, 1, ntotal-2, 0);
               power = sdf('f', critval, 1, ntotal-2, ncp);
               output;
            end;
         end;
      end;
   end;
run;

proc print data=tpow;
run;

The output is shown in Figure 18.6.

Figure 18.6 Customized Power Formula (DATA Step)

Obs	meandiff	stddev	alpha	ntotal	ncp	critval	power
1	5	12	0.05	100	4.3403	3.93811	0.54102
2	5	12	0.05	200	8.6806	3.88885	0.83447
3	5	12	0.10	100	4.3403	2.75743	0.66434
4	5	12	0.10	200	8.6806	2.73104	0.90171
5	5	18	0.05	100	1.9290	3.93811	0.27981
6	5	18	0.05	200	3.8580	3.88885	0.49793
7	5	18	0.10	100	1.9290	2.75743	0.39654
8	5	18	0.10	200	3.8580	2.73104	0.62287
9	6	12	0.05	100	6.2500	3.93811	0.69689
10	6	12	0.05	200	12.5000	3.88885	0.94043
11	6	12	0.10	100	6.2500	2.75743	0.79895
12	6	12	0.10	200	12.5000	2.73104	0.96985
13	6	18	0.05	100	2.7778	3.93811	0.37857
14	6	18	0.05	200	5.5556	3.88885	0.65012
15	6	18	0.10	100	2.7778	2.75743	0.50459
16	6	18	0.10	200	5.5556	2.73104	0.75935