Course Notes - Department of Mathematics and Statistics

8.3 Attributable Risk (AR)The attributable risk (AR) is given by w/(w+x) - y/(y+z).• So in our vaccine example:• Risk for being in vaccine group = 984• Risk for being in placebo group = 2285• Attributable Risk = 9 84 − 2285 = 0.15• We normally give the attributable risk as a whole number, so inthis example we would multiply it by 100, and say that in every100 people vaccinated there will be 15 fewer cases of flu, than ifthey had not been vaccinated.• Normally multiply by the smallest multiple of 10 that convertsour value to a whole number.8.4 Odds Ratio (OR)• Odds ratio (OR) gives the odds of an outcome relative to exposure.• It tends to be used in case-control studies (see why shortly)• It is calculated as a ratio of the odds of outcome for the exposedgroup to the odds of outcome for the unexposed group.OR = w/xy/z = wzxyDental enamel erosion example• Remember case-control is a retrospective study.• Is there an association between exposure to chlorinated water anddental enamel erosion?Erosion of EnamelSwim Time Per Week Yes (Cases) No (Controls) Total≥ 6 hrs 32 118 150< 6 hours 17 127 14449 245 294144