Compare the mean of a continuous measurement in two samples, using a z-statistic to approximate the t-statistic.
Instructions: Enter parameters in the red cells. Answers will appear in blue below.
The standard normal deviate for α = Zα =
The standard normal deviate for β = Zβ =
A = (1/q1 + 1/q0) =
B = (Zα+Zβ)2 =
Standardized Effect Size = (E/S) =
Total group size = N = AB/(E/S)2 =
Because the formula used here is based on approximating the t statistic with a z statistic, it will slightly underestimate the sample size when N is less than about 30.
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 6A, page 73.