Sample size for a descriptive study of a dichotomous variable.

This calculator gives both binomial and normal approximation to the sample size.

Instructions: Enter parameters in the red cells. The answer will appear in blue below.

CL = % Confidence level (e.g. 95%)
P = Expected proportion
W = Total width of confidence interval

1. Binomial "exact" calculation:

Sample size = N =

Expected positive results in sample = x =

2. Normal approximation to the binomial calculation:

α = (1-CL)/2 =

Standard normal deviate for α = Zα =

Sample size = N = 4Zα2P(1-P)/(W2) =

Expected positive results in sample = x =

Reference: Hulley SB, Cummings SR, Browner WS, Grady D, Newman TB. Designing clinical research : an epidemiologic approach. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013. Appendix 6E, page 81.