Table 3.

Relationship Between ICC, Cohen’s d, Success Rates and NNT

ICC	Cohen’s d50	Proportion of Untreated Controls Below Mean of Treated Persons	Success Rate of Untreated Persons	Success Rate of Treated Persons	NNT – Numbers Needed to Treat49
Small
0.0%	0.0	0.500	0.500	0.500	∞
0.2%	0.1	0.540	0.475	0.525	17.7
1.0%	0.2	0.579	0.450	0.550	8.9
Medium
2.2%	0.3	0.618	0.426	0.574	6.0
3.8%	0.4	0.655	0.402	0.598	4.5
5.9%	0.5	0.691	0.379	0.621	3.6
8.3%	0.6	0.726	0.356	0.644	3.0
10.9%	0.7	0.758	0.335	0.665	2.6
Large
13.8%	0.8	0.788	0.314	0.686	2.3

Notes: Cohen’s d’s aim is to describe the magnitude of response to treatments between two groups, for example, a treatment and a control group. More technically, “The difference between the Treatment and Control group means, divided by the within-group standard deviation”.⁵⁰ The Number Needed to Treat (NNT) is defined as the number of patients one would expect to treat with Treatment to have one more success (or one less failure) than if the same number were treated with Control.⁴⁹