In epidemiology, attributable risk is the difference in rate of a condition between an exposed population and an unexposed population.[1]
The concept was first proposed by Levin in 1953.[2][3]
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Diversity of interpretation
There is some variation in how the term is used.
The term population attributable risk (PAR) has been described as the reduction in incidence that would be observed if the population were entirely unexposed, compared with its current (actual) exposure pattern.[4] In this context, the comparison is to the existing pattern of exposure, not the absence of exposure.
There is some ambiguity in terminology. Population attributable risk is often simply called "attributable risk" (AR), and the latter term is most often associated with the above PAR definition. However, some epidemiologists use "attributable risk" when referring to the excess risk, also called the risk difference or rate difference.
Greenland and Robins distinguished between excess fraction and etiologic fraction in 1988.[5]
- Etiologic fraction is the proportion of the cases that the exposure had played a causal role in its development.
- It is defined as:[6]
- Where:
- EF = Etiologic fraction
- Ne = Number of exposed individuals in a population that develop the disease
- Nn = Number of unexposed individuals in the same population that develop the disease.
- Excess fraction, however, is the proportion of the cases that occurs among exposed population that is in excess in comparison with the unexposed.
All excess cases are etiologic cases, but not vice versa. From the standpoint of both law and biology it is important to measure the etiology fraction. In most epidemiological studies, PAR measures only the excess fraction. (Smaller than etiologic fraction)
Uses
Population attributable fraction estimates can help guide policymakers in planning public health interventions.[7] Population attributable fraction (PAF), population attributable risk proportion, and population attributable risk percent are all the same as PAR.
As a hypothetical example, if all the radon in a community were removed, and everything else was left unchanged, the number of lung cancer cases would fall. This fall is the population attributable risk for lung cancer from radon.
Combined PAR
The PAR for a combination of risk factors is the proportion of the disease that can be attributed to any of the risk factors studied. The combined PAR is usually lower than the sum of individual PARs since a diseased case can simultaneously be attributed to more than one risk factor and so be counted twice.
When there is no multiplicative interaction (no departure from multiplicative scale), combined PAR can be manually calculated by this formula:
Worked example
| Example 1: risk reduction | Example 2: risk increase | ||||
|---|---|---|---|---|---|
| Experimental group (E) | Control group (C) | Total | (E) | (C) | |
| Events (E) | EE = 15 | CE = 100 | 115 | EE = 75 | CE = 100 |
| Non-events (N) | EN = 135 | CN = 150 | 285 | EN = 75 | CN = 150 |
| Total subjects (S) | ES = EE + EN = 150 | CS = CE + CN = 250 | 400 | ES = 150 | CS = 250 |
| Event rate (ER) | EER = EE / ES = 0.1, or 10% | CER = CE / CS = 0.4, or 40% | N/A | EER = 0.5 (50%) | CER = 0.4 (40%) |
| Equation | Variable | Abbr. | Example 1 | Example 2 |
|---|---|---|---|---|
| EER − CER | < 0: absolute risk reduction | ARR | −0.3, or −30% | N/A |
| > 0: absolute risk increase | ARI | N/A | 0.1, or 10% | |
| (EER − CER) / CER | < 0: relative risk reduction | RRR | −0.75, or −75% | N/A |
| > 0: relative risk increase | RRI | N/A | 0.25, or 25% | |
| 1 / (EER − CER) | < 0: number needed to treat | NNT | (−)3.33 | N/A |
| > 0: number needed to harm | NNH | N/A | 10 | |
| EER / CER | relative risk | RR | 0.25 | 1.25 |
| (EE / EN) / (CE / CN) | odds ratio | OR | 0.167 | 1.5 |
| EE / (EE + CE) − EN / (EN + CN) | attributable risk | AR | (−)0.34, or (−)34% | 0.095, or 9.5% |
| (RR − 1) / RR | attributable risk percent | ARP | −300% | 20% |
References
- ^ "3. Comparing disease rates". http://www.bmj.com/epidem/epid.3.html. Retrieved 2009-04-19.
- ^ Paik, Myunghee Cho; Fleiss, Joseph L.; Levin, Bruce R. (2003). Statistical methods for rates and proportions. Hoboken, NJ: J. Wiley-Interscience. pp. 151. ISBN 0-471-52629-0.
- ^ Levin ML (1953). "The occurrence of lung cancer in man". Acta Unio Int Contra Cancrum 9 (3): 531–41. PMID 13124110.
- ^ Rothman K; Greenland S (1998). Modern Epidemiology, 2nd Edition. Lippincott Williams & Wilkins.
- ^ Greenland S; Robins JM. (1988). "Conceptual problems in the definition and interpretation of attributable fractions.". Am J Epidemiol. 128: 1185–1197. http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=3057878&cmd=showdetailview&indexed=google.
- ^ Page 43 in: Case control studies: design, conduct, analysis By James J. Schlesselman, Paul D. Stolley Edition: illustrated Published by Oxford University Press US, 1982 ISBN 019502933X, 9780195029338 354 pages
- ^ Northridge ME. (1995). "public health methods: attributable risk as a link between causality and public health action.". Am J Public Health 85: 1202–1203. doi:. PMID 7661224. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=7661224.
External links
- Population Attributable Risk Calculator (http://www.urmc.rochester.edu/smd/cpm/mach/pubs.html)
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