Risk Differences and Rate DifferencesRisk DifferencesInstead of comparing two measures of disease frequency by calculating their ratio, one can compare them in terms of their absolute difference. The risk difference is calculated by subtracting the cumulative incidence in the unexposed group (or least exposed group) from the cumulative incidence in the group with the exposure.where (CI e) = cumulative incidence among the exposed subjects, and (CI u) is the cumulative incidence among unexposed subjects.The risk difference focuses on absolute effect of the risk factor, or the excess risk of disease in those who have the factor compared with those who don't.
Hazard ratio (HR) Broadly equivalent to relative risk (RR); useful when the risk is not constant with respect to time. It uses information collected at different times. The term is typically used in the context of survival over time. If the HR is 0.5 then the relative risk of dying in one group is half the risk of dying in the other group. Ratio is the frequency of one item compared to another. Albumin-globulin ratio as a biochemical parameter and embolus-to-blood ratio for Doppler are examples. Epidemiologists use sex ratio and dependency ratio. In all ratios, the two items under comparison are different entities, and none is part of the other. RISK AND HAZARD.
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Recall that in the wound infection study, the cumulative incidence of infection was 5.3% in the incidental appendectomy group, and only 1.3% in the group without appendectomies. The risk ratio was 4.2, but we can also compute the absolute difference, which is 5.3/100 - 1.3/100 = 4 per 100 excess wound infections among those who had the incidental appendectomy. CI u provides an estimate of the baseline risk (i.e., in the absence of the exposure), and the exposure factor imposes an additional (excess) risk on top of that.An older term for the risk difference is 'attributable risk,' that is the excess risk than can be attributed to having had the exposure. Perspective of Relative Differences (Ratios) vs.
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