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09-15-2016 10:29 AM

I want to calculate sample size per group to test proportions if i think and response rate is 1% and i want to detect a difference of 0.5% with power 90%.

I run this:

proc power;

twosamplefreq test=pchi

groupproportions=(0.015, 0.01)

npergroup=.

power=.9;

run;

Is that correct for what i want to do? I get ~10,400 per group

I tried doing this for response rate 3% and detect a difference of 0.5% with 90% power. Logically i thought the Nper group would be smaller since the response rate is higher but its more. Which makes me think that i am doing something wrong.

Thanks

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09-15-2016 10:42 AM

Did perhaps want the NULLPDIFF option to dest the proportion difference? Set to 0.05 the sample size drops to 86.

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09-15-2016 10:49 AM

well i guess then i dont understand what the GROUPPROPORTIONS option means?

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09-15-2016 11:51 AM - edited 09-15-2016 01:11 PM

and to be honest with 1% response rate sample size of 86 does not feel reasonable...