# Sample size determination/verification

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.

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.

• 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 – user9292 Sep 29 '12 at 22:00
• 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. – Peter Flom Sep 29 '12 at 22:07