Instead of taking the effect size as the input and calculating the group sizes, this calculator takes the group sizes as inputs and calculates the effect size that the study has (1 - β) power to detect.
The effect size is calculated in two different ways: first using the T statistic (with a non-centrality parameter), then using the Z statistic. The Z statistic approximates the T statistic, but provides an effect size that is slightly too small. (We provide the Z statistic calcultion to allow comparison with other calculators which use the Z approximation.)
Instructions: Enter parameters in the red cells. Answer will appear in blue below.