Cohen’s d and Hedge’s g
For two Independent Means
1   2
d

    22   e2
Assuming
Power
 d
n
2
Then use the Power Table for deltas

Example
Mean
SD
n
Con1
10
18
20
Con2
15
20
20
Con3
25
22
20
Power for contrasting Condition 1 and condition 2
d
15  10
.25
20
 .25
20
.8
n
Power from table is approximately .15
How many subjects do we need for power = .8?
 d
n
2
 
n  2 
d
2
We need the delta that corresponds to power of .8 ( delta = 2.8 – from the table)
Our example from the previous page.
2
 2.8
2   250
 .25
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.
For Repeated Measures
d
1   
 x1 x 2
The denominator is the SD in the difference scores.
Variance Sum Law / Assuming Homogeneity of Variance
 x1 x 2      