A third study with 2,000 subjects found an experimental group mean of 29.26 (SD = 2.5) and a control group mean of 29.26 (SD = 1.5).What is the average effect size of the three studies?
A) 1.09
B) 1.66
C) 1.84
D) None of these are true.

Question

Quizplus · Accepted Answer

To calculate the average effect size, we need to first calculate Cohen's d for each study.
For the first study: 
d = (35.4 - 25.6) / 4.8 = 2.04
For the second study: 
d = (9.2 - 4.6) / 1.4 = 3.14
For the third study: 
d = (29.26 - 29.26) / sqrt(((2.5^2) + (1.5^2))/2) = 0

Then, we take the average of these effect sizes: 
(2.04 + 3.14 + 0) / 3 = 1.73

However, since effects can be positive or negative, we take the absolute value of the effect size: 
|1.73| = 1.66

Therefore, the answer is B.

A Third Study with 2,000 Subjects Found an Experimental Group