Evidenced-Based Formulas and Example
(Adapted from the original Chiefresidents.net site circa 2002)Formulas for evaluating a RCT (Randomized Control Trial)
EER = Experimental Event Rate
CER = Control Event Rate
Relative risk (RR) = EER/CER
Relative risk reduction (RRR) = 1-RR
Absolute Risk Reduction (ARR) = CER – EER
Number needed to treat (NNT) = (1/ARR)
Basic Example 1:
Lets say a study is done comparing the a standard blood thinner and an experimental thinner and the endpoint is cardiovascular events. The study shows that 11/100 patients in a control group develop a heart attack using the standard blood thinner (e.g. Aspirin). However, 9/100 patients in a experimental group develop a heart attack (using the experimental blood thinner).The Control Event Rate (CER) is 11/100 or 11%. The Experimental Event Rate is 9/100 or 9%. The Relative Risk is 9% divided by 11% = ~80%. The Relative Risk Reduction (RRR) = 1-RR = ~20%! (Wow, must be an excellent product).
The Absolute Risk Reduction (ARR) is a difference of the CER and the EER or 11% minus 9% = 2% (hmm..doesn't seem as impressive).
The Number needed to Treat (in other words, how many patients would need to be treated, before one adverse event could be prevented) is simply 1 divided by the ARR or 1 / 2% or 1/.02 = 50. That means we would have to treat 50 persons before 1 adverse event (heart attack) could be prevented.
Last Updated 11/4/02; 08/10/08 by Prashant Phatak, MD
