If you are a clinical researcher trying to determine how many subjects to include in your study or you have another question related to sample size or power calculations, we developed this website for you. Our approach is based on Chapters 5 and 6 in the 4th edition of Designing Clinical Research (DCR-4), but the material and calculators provided here go well beyond an introductory textbook on clinical research methods.

DCR-4 Chapter 6 “Estimating Sample Size and Power” Appendixes A-F provide instructions and tables for calculating sample sizes. However, the tables provided for two-group studies assume that the groups are of equal size. If you have unequal group sizes (as occurs in most observational studies) or a study parameter that does not correspond to one of the table headings, then you may have to use one of these online calculators.

#### Online Calculators Corresponding to Tables in DCR-4 Chapter 6

**Two-group analytic study comparing**

(A) Means

(A1) sample size given effect size

(A2) effect size given sample size

(A’) Means with clustered sampling

(A’1) sample size given effect size (clustered sampling)

(A’2) effect size given sample size (clustered sampling)

(B) Proportions

(B1) sample size given both proportions or baseline proportion and relative risk or odds ratio

(B2) second proportion (and relative risk and odds ratio) given baseline proportion and sample sizes

**One-group analytic study comparing**

(C) the correlation coefficient to zero (or another specified value)

(C1) sample size given correlation coefficient(s)

(C2) correlation coefficient given sample size (Coming Soon)

**One-group descriptive study estimating a**

(D) mean

(D1) sample size given the width of the confidence interval

(D2) confidence interval given the sample mean and the sample size

(E) proportion

(E1) sample size given the expected proportion and the width of the confidence interval

(E2) confidence interval given the sample proportion and the sample size

**One-group before-after study comparing the**

(F) mean change score to zero

(F1) sample size given effect size and the standard deviation of the change score

(F2) effect size given sample size and the standard deviation of the change score

#### Online Calculators Corresponding to Other Sample Size Problems

**Two-group survival analysis calculating**

(I) hazard ratio

(I1) sample size given hazard ratio and baseline event rate

(I2) hazard ratio given sample size and baseline event rate (Coming Soon)