Risk difference: Difference between revisions
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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.<ref>{{cite web| url=http://www.medepi.net/meta/lectures/An_Overview_of_Measurements_in_Epidemiology_V2_2003.pdf |title=An overview of measurements in epidemiology |accessdate=2010-02-01}}</ref> It is the inverse of the [[number needed to treat]].<ref>{{cite journal | last1 = Laupacis | first1 = A | last2 = Sackett | first2 = DL | last3 = Roberts | first3 = RS | title = An assessment of clinically useful measures of the consequences of treatment. | journal = The New England journal of medicine | volume = 318 | issue = 26 | pages = 1728–33 | year = 1988 | pmid = 3374545 | doi = 10.1056/NEJM198806303182605 }}</ref> |
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.<ref>{{cite web| url=http://www.medepi.net/meta/lectures/An_Overview_of_Measurements_in_Epidemiology_V2_2003.pdf |title=An overview of measurements in epidemiology |accessdate=2010-02-01}}</ref> It is the inverse of the [[number needed to treat]].<ref>{{cite journal | last1 = Laupacis | first1 = A | last2 = Sackett | first2 = DL | last3 = Roberts | first3 = RS | title = An assessment of clinically useful measures of the consequences of treatment. | journal = The New England journal of medicine | volume = 318 | issue = 26 | pages = 1728–33 | year = 1988 | pmid = 3374545 | doi = 10.1056/NEJM198806303182605 }}</ref> |
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For instance, 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. |
For instance consider Otegahs, consider a hypothetical drug which reduces the [[relative risk]] of [[colon cancer]] by 50% over |
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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. |
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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 [[probability|probabilities]] ''p<sub>A</sub>'' and ''p<sub>B</sub>'' of this endpoint under treatments ''A'' and ''B'', respectively, are known, then the absolute risk reduction is computed as (''p<sub>B</sub>'' - ''p<sub>A</sub>''). |
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 [[probability|probabilities]] ''p<sub>A</sub>'' and ''p<sub>B</sub>'' of this endpoint under treatments ''A'' and ''B'', respectively, are known, then the absolute risk reduction is computed as (''p<sub>B</sub>'' - ''p<sub>A</sub>''). |
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Revision as of 11:02, 8 August 2011
 | It has been suggested that Excess risk be merged into this article. (Discuss) Proposed since February 2010. |
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 instance consider Otegahs, 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 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.
Presenting results
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 example
Gape Lebella
References
- ^ "An overview of measurements in epidemiology" (PDF). Retrieved 2010-02-01.
- ^ Laupacis, A; Sackett, DL; Roberts, RS (1988). "An assessment of clinically useful measures of the consequences of treatment". The New England journal of medicine. 318 (26): 1728–33. doi:10.1056/NEJM198806303182605. PMID 3374545.
- ^ Ben Goldacre (2008). Bad Science. New York: Fourth Estate. pp. 239–260. ISBN 0-00-724019-8.
External links
- Measures of effect size of an intervention - unmc.edu.