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:

- where α = alpha, β = 1 - power, ψ = odds ratio, n_c is the continuity corrected sample size, m is the number of control subjects per case, and z_p is the standard normal deviate for probability p. n is rounded up to the closest integer.