The document discusses various measures used to assess the strength and nature of associations between variables in epidemiological studies. It describes difference measures like absolute risk and ratio measures like relative risk and odds ratio. It explains how relative risk is calculated in cohort studies and how odds ratio is used as a measure of association in case-control studies. The relationship between relative risk and odds ratio is also covered.

1. MEASURES OF ASSOCIATION Presenter: P. Ganeshkumar Moderator: Dr. Pragati Chhabra

3. ASSOCIATION 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI.

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15. Types of measures of association used in analytic epidemiologic studies. 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. Type Example Usual application Absolute difference AR (Attributable Risk) Primary prevention impact: search for causes. PAR(Population Attributable risk) Primary prevention impact Efficacy Impact of intervention on recurrences, case fatality etc. Mean differences (continuous outcome) Search for determinants Relative difference Relative risk/rate Search for causes Relative odds (ODDS ratio) Search for causes

17. Relative risk of developing the disease is expressed as the ratio of the risk(incidence) in exposed individuals (q+) to that in unexposed individual (q-) Total exposed = a+b Total unexposed = c+d 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. Diseased Nondiseased Exposed a b Unexposed c d

19. Hypothetical cohort study of the one year incidence rate of acute MI. Relative risk = IE/IU Incidence among exposed = 180/10000 = 0.0180 Incidence among unexposed = 30/10000=0.0030 RR = 0.0180/0.0030 = 6.00 ar 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. OR/RR Myocardial infarction Blood pressure Present Absent Total Severe hypertension 180(a) 9820(b) 10000 Normal 30(c) 9970(d) 10000

20. Interpreting Relative risk of a disease. 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. RR = 1 No association RR > 1 Positive association (possibly causal) RR < 1 Negative association (possibly protective)

21. Framingham study during first 12 years 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. Serum cholesterol Men Women 30-49 yr 50-62 30-49 50-62 Incidence rates (per 1000) <190 38.2 105.7 11.1 155.2 190-219 44.1 187.5 9.1 88.9 220-249 95.0 201.1 24.3 96.3 250+ 157.5 267.8 50.4 121.5 Relative risks <190 1.0 2.8 0.3 4.1 190-219 1.2 4.9 0.2 2.3 220-249 2.5 5.3 0.6 2.5 250+ 4.1 7.0 1.3 3.2

31. Calculation of the odds ratio is based on discordant pairs. Odds ratio (matched pairs) = b/c 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. Concordant pairs Pairs in which both the case and controls were exposed. (a) Pairs in which neither the case nor the control were unexposed (d) Discordant pairs Pairs in which the case was exposed but the control was not (b) Pairs in which the control was exposed and the case was not. (c) CASE- CONTROL PAIRS CONTROL Exposed Unexposed CASE Expsoed a b Unexposed c d

37. Incidence of local reactions in the vaccinated and placebo groups, influenza vaccination trial. Seltser et al 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. Local reaction Group Present Absent Total Probability Probability odds Vaccine 650 1920 2570 650/2570= 0.2529 650/(2570-650) =650/1920 = 0.3385 Placebo 170 2240 2410 170/2410=0.0705 170/(2410-170) = 170/2240 = 0.0759

41. Relationship between RR and OR by the incidence of the outcome. 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI.

47. %AR in case-control studies This formula can be used in case-control studies , in which the incidence data are unavailable, but the odds ratio can be used as an estimate of the relative risk if the disease is relatively rare. 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI.

49. AR exp BACKGROUND RISK 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI.

51. Smoking and heart disease: Hypothetical cohort study of 3000 cigarette smokers and 5000 non-smokers Assuming the proportion of smokers in the pop. P e : 44% Therefore the proportion of non-smokers (1- p e ): 56% 21.3% of the incidence of CHD in the total population can be attributed to smoking. If an effective prevention program eliminated smoking, the best that we could hope to achieve would be reduction of 21.3% in the incidence of CHD in the total population consisting of both smokers and non-smokers. 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. CHD develops CHD does not develop Total Incidence per 1000 per year Smoke 84 2916 3000 ( q+ )28 Do not smoke 87 4913 5000 (q-)17.4

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67. REFERENCES 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI. 3.Measures of Association and Hypothesis Testing by Deborah Rosenberg, PhD and Arden Handler, DrPH 4.Causation and Causal Inference in Epidemiology Kenneth J.Rothman, DrPH, Sander Greenland, MA, MS, DrPH, C Stat

68. Hypothesis testing for RR 24-Dec-08 DEPT. OF COMMUNITY MEDICINE, UCMS&GTBH DELHI.