This article is about an epidemiological concept. For other uses, see ARR.

In epidemiology, the absolute risk reduction or risk difference is the decrease in risk of a given activity or treatment in relation to a control activity or treatment.^[1] It is the inverse of the number needed to treat.^[2]

For example, consider a hypothetical drug which reduces the relative risk of colon cancer by 50% over five years. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every five-year period. The rate of colon cancer for a five-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.

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 p_A and p_B of this endpoint under treatments A and B, respectively, are known, then the absolute risk reduction is computed as (p_B - p_A).

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.

Presenting resultsEdit

The raw calculation of absolute risk reduction is a probability (0.003 fewer cases per person, using the colon cancer example above). Authors such as Ben Goldacre believe that this information is best presented as a natural number in the context of the baseline risk ("reduces 2 cases of colon cancer to 1 case if you treat 6,000 people for five years").^[3] Natural numbers, which are used in the number needed to treat approach, are easily understood by non-experts.

Worked exampleEdit

Template:ARR RRR worksheet

ReferencesEdit

1. ↑ An overview of measurements in epidemiology. URL accessed on 2010-02-01.
2. ↑ Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 1988;318:1728-33. PMID 3374545.
3. ↑ Ben Goldacre (2008). Bad Science, 239–260, New York: Fourth Estate.

See alsoEdit

• Absolute risk increase
• Number needed to harm

External linksEdit

• Measures of effect size of an intervention - unmc.edu.

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