Estimating the size of the treatment effect
Outcome
Outcome
risk of
does not
occurs
outcome
occur
a
b
Y = a/(a + b)
EER
(experimental
event rate)
Control
Unexposed c
(X)
d
X = c/(c + d)
CER (control
event rate)
Treatment
Exposed
(Y)
Clinical Significance calculator
Relative Risk (RR) is the risk of the outcome in the exposed (treatment) group divided by the risk of the
outcome in the unexposed (control) group.
RR = Y / X
Relative Risk Reduction (RRR) is the percent reduction in risk in the treated group compared to the
control group.
RRR = [ ( X - Y ) / X ] x 100%
Absolute Risk Reduction (AAR) is the difference in risk between the control group and the treatment
group.
AAR = X - Y
Number Needed to Treat (NNT) is the number of patients that must be treated over a given period of
time to prevent one adverse outcome.
NNT = 1/ARR = 1 / ( X - Y )
"An NNT of 15 means that 15 patients must be treated over a given period of time to prevent one
adverse outcome."
Reference:
Sackett, D. Evidence-Based Medicine: How to Practice and Teach EBM. 2nd ed. (CD-ROM) Churchill
Livingstone, 2000.
ACP Journal Club 1994 Jul-Aug:A10-A12.