Sample size for independent case-control studies

Menu location: Analysis_Sample Size_Independent Case-Control.

This function gives the minimum number of case subjects required to detect a real odds ratio or case exposure rate with power POWER and two sided type I error probability ALPHA. This sample size is also given as a continuity-corrected value intended for use with corrected chi-square and Fisher's exact tests (Schlesselman, 1982; Casagrande et al. 1978; Dupont, 1990).

Information required

POWER: probability of detecting a real effect
ALPHA : probability of detecting a false effect (two sided: double this if you need one sided)
P0: probability of exposure in controls
*Input either P1 or OR
P1: probability of exposure in case subjects
OR : odds ratio of exposures between cases and controls
M : number of control subjects per case subject

Practical issues

Usual values for POWER are 80%, 85% and 90%; try several in order to explore/scope.
5% is the usual choice for ALPHA.
P0 can be estimated as the population prevalence of exposure.
If possible, choose a range of odds ratios that you want have the statistical power to detect.

Technical validation

The estimated sample size n is calculated as:

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$image\STAT0082_wmf.gif$

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- where a = alpha, b = 1 - power, y = odds ratio, nc is the continuity corrected sample size and Zp is the standard normal deviate for probability p. n is rounded up to the closest integer.