statistics
posted by marty .
A residential treatment facility tests a new group therapy for patients with self destructive behaviors. To decrease scores of behaviors that has a mean in the overall population of 35 and a standard deviation of 4.7. The mean score for patients after new group therapy is 27. What is effect size of the new group therapy.

Cohen's d is the difference between two means divided by a standard deviation.
Calculate Cohen's d (d) and the effectsize correlation (r) using the following formulas:
d = (M1  M2) / s
r = d / √(d^2 + 4)
With your data:
d = (35  27) / 4.7 = 1.7 (rounded)
r = 1.7 / √(1.7^2 + 4) = 0.65 (rounded)
Check these formulas and calculations.
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