Sample size calculation: Paired samples t-test
Paired samples t-test
Calculates the required sample size for the paired samples t-test. The sample size takes into account the required significance level and power of the test (see Sample size calculation: Introduction).
- Type I error - alpha: the probability of making a Type I error (α-level, two-sided), i.e. the probability of rejecting the null hypothesis when in fact it is true.
- Type II error - beta: the probability of making a Type II error (β-level), i.e. the probability of accepting the null hypothesis when in fact it is false.
- Mean difference: the hypothesized mean difference (considered to be biologically significantly different from the null hypothesis value 0).
- Standard deviation: hypothesized standard deviation of differences (known for example from a Paired samples t-test from previous studies, or from the literature).
You consider an average difference between two paired observations before and after a study, of at least 8 to be meaningful. From a previous study you expect the standard deviation of the differences to be 12.
For α-level you select 0.05 and for β-level you select 0.20 (power is 80%). For mean difference, you enter 8 and for standard deviation enter 12.
After you click the Calculate button the program displays the required number of data pairs (20 in the example).
A table shows the required sample size for different Type I and Type II Error levels.
- Machin D, Campbell MJ, Tan SB, Tan SH (2009) Sample size tables for clinical studies. 3rd ed. Chichester: Wiley-Blackwell.