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Module appendix - Attributable risk

Principles of Epidemiology for Public Health (EPID600). Module appendix - Attributable risk. Victor J. Schoenbach, PhD home page Department of Epidemiology Gillings School of Global Public Health University of North Carolina at Chapel Hill www.unc.edu/epid600/. Attributable risk - conceptual.

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Module appendix - Attributable risk

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  1. Principles of Epidemiology for Public Health (EPID600) Module appendix - Attributable risk Victor J. Schoenbach,PhD home page Department of EpidemiologyGillings School of Global Public HealthUniversity of North Carolina at Chapel Hill www.unc.edu/epid600/ Attributable risk

  2. Attributable risk - conceptual Assume that we know that a factor causes a disease. How can we answer the “so what?” questions: • “How much risk is attributable to that factor?” • “How many cases are attributable to that factor?” The answers depend on: (1) by how much the factor increases risk and (2) how common the risk factor is. A strong risk factor that is common has a large impact. Attributable risk

  3. Background risk – 8 cases / 200 people(1/25 = 4%) O    O O O      O O O O Attributable risk

  4. Risk in exposed vs. unexposed (4/75 = 5.3%) vs. (5/125 = 4%) O    O O O O      O O O O Attributable risk

  5. Attributable risk – proportion of risk in exposed population (PAR) O    O O O O      O O O O Attributable risk

  6. Attributable risk – proportion of risk in the entire population (PARP) O    O O O O      O O O O Attributable risk

  7. Attributable risk – counterfactual comparison O    O O O O      O O O O Attributable risk

  8. Attributable risk measures Attributable risk

  9. Attributable risk – substitute population required 4 cases in 75 exposed persons, risk = 5.33% 5 cases in 125 unexposed persons, risk = 4.00% 9 cases in 200 persons (total), risk = 4.50% Attributable risk

  10. Attributable risk calculations 4 cases in 75 exposed persons, risk = 5.33% 5 cases in 125 unexposed persons, risk = 4.00% 9 cases in 200 persons (total), risk = 4.50% Attributable risk = 5.33% − 4.00% = 1.33% (risk difference) Population attributable risk (PAR) = 4.50% − 4.00% = 0.50% Attributable risk proportion (ARP) = 1.33% / 5.33% = 25% (attributable cases / exposed cases = 1/4 = 25%) Population attributable risk proportion (PARP) = 0.50% / 4.50% = 11% (attributable cases / all cases = 1/9 = 11%) Attributable risk

  11. Attributable risk – assumption, perspectives We are assuming that the exposure is a cause of the disease. The “attributable risk” for an exposure is the risk that would not have occurred without the exposure. Can use either of two perspectives: 1. difference in risk between exposed and unexposed people 2. difference in risk between total population and unexposed people Attributable risk

  12. Attributable risk – relative or absolute Attributable risk can be presented as: 1. an “absolute” number, e.g., “80,000, or 20 per 100 cases/year of stroke are attributable to smoking” 2. a “relative” number, e.g., “20% of stroke cases are attributable to smoking”. (analogy: a wage increase in a part-time job: $ increase, % increase in wage, % increase in income) Attributable risk

  13. How much risk is attributable: absolute perspective Incidence proportion (risk) People Attributable risk

  14. How many cases are attributable? (absolute) Incidence proportion (risk) How many cases are expected without the exposure? (2.5% x 3,000 = 75) 50 25 People Attributable risk

  15. Absolute perspective: amount of caseload Incidence proportion (risk) How many cases are attributable? (150 – 75 = 75) 75 25 50 People Attributable risk

  16. For relative measures, think of % of cases Cases Attributable risk

  17. Relative perspective: % of caseload Incidence proportion (risk) Express attributable cases as a % of all exposed cases “Attributable risk proportion or %” 75% People Attributable risk

  18. Relative perspective: % of caseload Incidence proportion (risk) Express attributable cases as a % of all cases “Population attributable risk proportion or %” 50% Attributable risk

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