Power and Sample Size Requirements, Cohort Studies

Use either StatCalc* or EpiTable* to perform calculations.

The determinants of power and sample size are:

• the [allocation] ratio of unexposed to exposed subjects (n₂  / n₁)
• the expected relative risk (a RR "worth detecting")
• the expected incidence in unexposed subjects (p₂), and
• the alpha (confidence) level of the test.

Of course, power and sample size are inversely related.

* These computer program uses formulas from Fleiss (1981, pp. 44 - 45).