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I need to calculate the sample size required for an observational study in which incidence of disease is 19-29%. Population affected is 600,000 people.

The study has two samples similar on baseline characteristics treated with two different drugs.

Statistical analysis will be chi-square and Fisher's test. I have to demonstrate non-inferiority of one drugs with respect of the other one. I don't have any other info.

Can anyone help me to calculate the sample sizes required in order to obtain power of 0.80 with an alpha 0.05?

Thanks a lot in advance

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Surely one of the applets at statpages.org/#Power will solve your problem. – whuber♦ Dec 14 '10 at 18:35
@Roland How could you compute a sample size in a power study from a non-inferiority trial if you don't know the expected difference (or equivalently, the % of patients responding to the new treatment compared to the actual one)? – chl♦ Dec 15 '10 at 20:35

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G*Power is a commonly-recommended program for sample size calculations. I've only dabbled with it a couple of times in the past, but it's more than capable of handling the situation you describe.

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(+1), excellent program. To spare the reader an extra click, you could link to the new version (G*Power3) which has a GUI for Windows and MacOS X: psycho.uni-duesseldorf.de/abteilungen/aap/gpower3 – caracal Dec 14 '10 at 20:50
Oops - thanks for that, caracal. I just grabbed the link without noticing the content of the page. – Matt Parker Dec 15 '10 at 2:34
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Are all your participants infected? If you also have a placebo group and your df= 3 then according to Cohen (1992) at power = .8 and alpha = 0.5, for a chi square analysis will need 40 (39) participants if your effect is strong, 107 if it is a medium effect zise and 964 if its only a small effect. For only 2 groups the sizes are 26;87;725 for strong medium and small effect sizess respectively. see: http://classes.deonandan.com/hss4303/2010/cohen%201992%20sample%20size.pdf for the article

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