1. For two Independent Means
Cohen’s d and Hedge’s g
For two Independent Means
Assuming
Power
Then use the Power Table for deltas
Example
Con1 Con2 Con3
Mean SD n
Power for contrasting Condition 1 and condition 2
Power from table is approximately .15

2. How many subjects do we need for power = .8?
We need the delta that corresponds to power of .8 ( delta = 2.8 – from the table)
Our example from the previous page.
Thus, we would require approximately 250 subjects per condition to have a power of .8
Remember, there is only an issue of power if the null hypothesis is false.

3. Variance Sum Law / Assuming Homogeneity of Variance
For Repeated Measures
The denominator is the SD in the difference scores.
Variance Sum Law / Assuming Homogeneity of Variance