Absolute risk reduction
In general, absolute risk reduction is usually computed with respect to two treatments A and B, with A typically a drug and B a placebo (in our example above, A is a 5-year treatment with the hypothetical drug, and B is treatment with placebo, i.e. no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilities pA and pB of this endpoint under treatments A and B, respectively, are known, then the absolute risk reduction is computed as pB-pA.
The inverse of the absolute risk reduction, NNT, is an important measure in pharmacoeconomics. If a clinical endpoint is devastating enough (e.g. death, heart attack), drugs with a low absolute risk reduction may still be indicated in particular situations. If the endpoint is minor, health insurers may decline to reimburse drugs with a low absolute risk reduction.
For example, consider a hypothetical drug which reduces the risk of colon cancer by 50%. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every 5 year period. The absolute risk reduction for a 5-year treatment with the drug is therefore 1/6,000, as by treating 6,000 people with the drug, one can expect to reduce the number of colon cancer cases from 2 to 1.
|-||subjects in control group||-||250|
|-||subjects in experimental group||-||150|
|-||events in control group||-||100|
|-||events in experimental group||-||15|
|CER||control event rate||= events / subjects in control group||0.4, or 40%|
|EER||experimental event rate||= events / subjects in experimental group||0.1, or 10%|
|ARR||absolute risk reduction (or increase)||= CER - EER||0.3, or 30%|
|RRR||relative risk reduction (or increase)||= (CER - EER) / CER||0.75|
|NNT||number needed to treat/number needed to harm||= 1 / ARR||3.33|
|OR, RR||odds ratio, relative risk (not really identical, but similar -- see articles for details)||= CER / EER||4|
- Laupacis A, Sackett DL, Roberts RS (1988) An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 318 (26):1728-33. DOI:10.1056/NEJM198806303182605 PMID: 3374545