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Sample Size Calculation

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Contributor
Posts: 60

Sample Size Calculation

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

Super User
Posts: 11,343

Re: Sample Size Calculation

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

Contributor
Posts: 60

Re: Sample Size Calculation

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

 

Contributor
Posts: 60

Re: Sample Size Calculation

[ Edited ]

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

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