东南大学流行病学课件.ppt

胃癌,胃切除 * Exploratory case-control studies have been referred to in a disparaging manner as “fishing expeditions” but they can be valuable in identifying potential risk factors. * * * * Also: cumulative incidence Rate of contracting diesease among those still at risk Individuals contribute with different lengths of time Denominator = sum of times * Can we use a sample of the denominator instead of the entire denominator? * * Cornfield (1961): if disease is rare OR ~ RR * * Fixed population may include cases A person selected as a case may also be selected as a control and vice versa No need to document disease status among controls Example: outbreak of gastro-enteritis with 30% attack rate * * * Incidence risk 5% * * * ORcrude=2.3 (1.4-3.9) * Cases Exposed Unexposed Source population Sample Controls Cases = the same as in cohort study Controls = sample of the source population, with representative distribution of exposed and unexposed persons (or person-time) Cases Exposed Unexposed Source population Sample Controls a d c b Cases Exposed Unexposed Source population Sample Controls a d c b d / Nt0 = c / Nt1 because sampled independent of exposure 24 / 120 = 25 / 125 The case-control study: calculation IRR = I1 = a / Nt1 I0 b / Nt0 = a . Nt0 b Nt1 = a . d b c d = c Nt0 Nt1 Nt0 = d Nt1 c = 16 . 24 = 1.9225 8 Saving resources with case-control study In stead of following a cohort of 245 people for one year to wait for the 24 cases We investigated the 24 cases in order to divide them between exposed a = 16 and unexposed b = 8 We chose 49 controls and investigated them in order to divide them between exposed c = 25 and unexposed d = 24 The result exactly the same as cohort study, but much easier The odds ratio (OR) OR = Incidence rate ratio (IRR) OR = Risk ratio (RR) Cross product ratio: ad / bc Summary of the case-control study Imagine a source population Consists of exposed and unexposed people Gives rise