Sample size calculation: Comparison of two means
Calculates the required sample size for the comparison of two independent means. The sample size takes into account the required significance level and power of the test (see Sampling: Introduction).
Correction for unequal variances
When you enter the same standard deviation for both samples, it is assumed that the data will be analysed using the Independent samples t-test with the option "Assume equal variances". When you enter two different standard deviations, it is assumed that the data will be analysed using the Independent samples t-test with the option "Assume unequal variances". See Independent samples t-test.
In the example you are interested in detecting a difference between the sample means of a least 10. You expect the standard deviations in the two studies to be equal to 16. You expect to include twice as many cases in group 1 as in group 2.
For α-level you select 0.05 and for β-level you select 0.20 (power is 80%).
After you click the Calculate button the program displays the required sample size, which is 61 for group 1 and 31 in group 2, or a total of 92 cases.
A table shows the required sample size for different Type I and Type II Error levels.
Comparison of two paired samples
To calculate the sample size required for the comparison of two paired samples, see Sampling for single mean.