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### 14 Cards in this Set

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 Attributable risk and risk difference the difference between the incidence rates in exposed and non-exposed groups. In a cohort study, AR is calculated as the difference in cumulative incidences (risk difference) or incidence densities (rate difference). This reflects the absolute risk of the exposure or the excess risk of the outcome (e.g. disease) in the exposed group compared with the non-exposed group. AR is sometimes referred to as attributable risk in the exposed because it is used to quantify risk in the exposed group that is attributable to the exposure. AR= I(e)-I(pop) Population attributable risk (PAR) different from AR. PAR estimates the proportion of disease in the study population that is attributable to the exposure. In order to calculate PAR, the incidence of exposure in the study population must be known or estimated. PAR=I(pop)-I(not exposed) But we hardly ever know I(pop), so use proportion exposed. I(pop)=I(e)P(e)+I(ne)P(ne) Attibutable Risk % (or fraction) proportion of incidence among the exposed that is attributable to the exposure AR%=AR/I(e) Population attributable risk Percent (%) The proportion of the incidence in the total population that is attrributable to exposure. PAR%=PAR/I(pop). Ask "How much total reduction in population can we expect?" AR and PAR in CCS? NO! cannot be calculated directly. The incidence rate in exposed and non-exposed are necessary. BUT is OK if have OR (RR) and proportion exposed in population. CCS + AR% and PAR% AR%=(OR-1)/OR x 100 PAR% = [(P(e))(OR-1)]/{[(P(e))(OR-1)]/{[(Pe)(OR-1)]+1 If proportion exposed in the control group estimates Pe then PAR%=AR% x Pe In CCS Incidence is not available directly. Incidence in the exposed = RR(I(ne)) = OR(I(ne)) So AR%=(OR-1)/OR Asks "What proportion of disease is due to exposure?" PAR% {(Pe)(RR-1)/{(Pe)(RR-1)}+1 Ask, "What percent of the disease in the population could be prevented by removing the exposure" DO NOT ADD RR of different exposures. NEVER NEVER NEVER I(e) I(ne) NEVER = I(pop) Risk Difference=Attributable risk AR Attributable fraction = AR% or proportion in the exposed AR% Population risk difference =Attributable risk for the total population PAR Population attributable fraction = AR proportion in the total population PAR% Causality 1. Plausibility (known path) 2. Consistency (same results if repeat in different time, place person) 3. Temporal relationship 4. Strength (with or without a dose response relationship) 5. Specificity (causal factor relates only to the outcome in question - not often) 6. Change in risk factor (i.e. incidence drops if risk factor removed)