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 green cells. Answers will appear in the blue box below.

P =

Expected proportion

W =

Total width of confidence interval

Confidence level

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 = Z_α =

Sample size = N = 4Z_α²P(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.