I need to make a power analysis calculation to estimate the minimum sample size for a relative risk statistic (ratio of two proportions) for unmatched data - two independent samples.

Can I use the power calculations in the R pwr package for testing the difference between two proportions? Or will this return inaccurate sample size estimates?

EDIT: I see there's a power.cmh.test option in the samplesizeCMH package that allows you to find power and sample size for Cochran-Mantel-Haenszel tests. Can I apply this to any relative risk statistic, even if it's not a case-control study?


The relative risk, $RR$, is simply $RR = \pi_1/\pi_2$ for group probabilities $\pi_i$. This means that $\pi_1 = RR \pi_2$.

Thus, a power analysis to detect a relative risk of $RR$ is equivalent to a power analysis to detect a risk difference between $\pi_1$ and $RR \pi_2$. See chapter 3 in Biostatistical Methods 2e by John Lachin for more.

  • $\begingroup$ Thanks Demetri! $\endgroup$
    – RobertF
    Sep 14 at 14:06

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