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I am going to conduct a survey in order to collect information for two variables and then calculate the correlation between them.

If we assume that $alpha$ = 0.05, $power=0.80$. Can someone show me a step by step method of how a sample of 85 was determined with a medium effect size? And is there a procedure in SAS I can use?

Thanks.

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In SAS you can use PROC POWER. Something like

proc power;
   onecorr dist=fisherz
      nullcorr = 0.00
      corr = 0.10 to .9 by .1
      ntotal = .
      power = .8;
run;

seems to do what you want. Definitions of 'medium' can, of course, vary; many people use the definition suggested by Jacob Cohen.

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  • $\begingroup$ I ran the code, and here is a line from output: r= 0.3, alpha=0.0500, power=0.800, n=84. The sample size is 84, but I'm a bit confused about r=0.3, I don't know what's the value of r, but here it's specified to be 0.3. Can you please explain this? thanks $\endgroup$ – user9292 Sep 29 '12 at 22:00
  • $\begingroup$ r is the correlation coefficient. That is, if you are testing if a correlation is different from 0 - nullcorr = 0.00 - with power = .8 and alpha = .05 (default) then the minimum sample size is 84. $\endgroup$ – Peter Flom Sep 29 '12 at 22:07

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