Cohort and case-control studies

1. Cohort and case-control studies Observational studies

2. Learning Objectives • At the end of the sessions, the participants will be able to: • Define cohort and case-control studies • Identify applications for cohort and case-control studies • Describe two-by-two table • Compute different measures of association

3. Cohort study Observational studies: Cohort and case-control studies

4. Learning Objectives • At the end of the session, the participants will be able to: • Define cohort study • Describe prospective and retrospective cohort studies • Compute attack rate and relative risk • Enumerate pros and cons in using cohort studies

5. What is a cohort? • Cohort: Latin word for one of the 10 divisions of a Roman legion • A group of individuals • Sharing same experience • Followed-up for a specified period of time • Examples • Birth cohort • Occupational cohort chemical plant workers • A Rapid Response Team

6. Prospective Cohort Study ill + - + exp - Study starts Exposure occurrence Disease occurrence + exp - Time Selection of population Prospective assessment of exposure and disease Growth-nutrition studies, Folic acid and NT defects

7. Prospective cohort study ill + - + + exp exp - - Exposure occurrence Study starts Disease occurrence Selection based on exposure Prospective assessment of disease Chernobyl, Industrial accidents, Flood victims

8. Retrospective cohort study Transversal studies ill + - + exp - Now Real Time Exposure occurrence Disease occurrence Study takes place Retrospective assessment of exposure and disease Selection based on population Food borne outbreaks, closed environment outbreaks (school, prisons, etc)

9. Presentation of Cohort Datatwo-by-two table ill not ill a b Exposed c d Unexposed Incidence in exposed (Ie+) = a/(a+b) Incidence in non-exposed (Ie-) = c/(c+d)

10. Effect measures in cohort studies • Ie+ • Ie- • a/(a+b) • c/(c+d) • = • Hypothesis • Is the incidence among exposed higher than among unexposed • Absolute measures • Risk difference (RD) Ie+ - Ie- • Relative measures • Relative risk (RR) Rate ratio Risk ratio

11. Foodborne Outbreak in a Wedding, Dublin not ill Incidence ill 49 49 98 50% Ate ham Did not eat ham 4 6 10 40% 53 55 108 Risk difference 0.5 - 0.4 = 0.1 (10%) Relative risk 0.5 / 0.4 = 1.25

12. Presentation of cohort data Population at risk Does HIV infection increase the risk of developing TB among a population of drug users? Drug users (f/u 2 years) TB Cases Relative risk Incidence (%) HIV + 215 8 3.7 (8/215) 12 HIV - 289 1 0.3 (1/289) Source: Selwyn et al., New York, 1989

13. Advantages and disadvantages of cohortstudies Advantages Can measure incidence and risks Good for rare exposures Clear temporal relationship between exposure and outcome Less subject to selection bias Disadvantages Requires a large sample size Latency period Lost to follow-up Ethical considerations Resource intensive High cost Timely

14. Case-Control study Observational studies: Cohort and case-control studies

15. Learning Objectives • At the end of the session, the participants will be able to: • Define case-control study • Describe steps in setting up a case-control study • Compute Odds Ratio • Enumerate pros and cons in using case-control studies

16. Case-Control Study ill ill + - + - + exp - Now Real Time Exposure occurred Disease occurred Study takes place Retrospective assessmentof exposure Selection based on diseasestatus

17. Setting-up a case-control study • Identify group of cases • Identify group of controls • Question both groups for possible exposure • Measure the frequency of exposure occurrence in both groups • Compare the frequency of exposure between cases and controls

18. Presentation of case-control datatwo-by-two table case control a b Exposed c d Not exposed % of exposure among cases = a/(a+c) % of exposure among controls = b/(b+d)

19. Odds of exposure cases controls 18 14 Exposed 19 43 Not exposed 37 57

20. Effect measures in case-control studies • a/c • b/d • a*d • b*c • OR • = • = • OR = 1 – no association • OR > 1 – there is an association • OR < 1 – the factor is some way protective

21. Distribution of Cholera Cases and Controls by Exposure to Shallow Well WaterN'Djamena, July 2001 Drink water From well Cholera Control Yes 30 25 No 32 60 Total 62 85 % exposed 48.4% 29.4 % 30 * 60 OR = ----------- = 2.25 (95%CI 1.07-4.75) 32 * 25

22. Multiple Exposure Levels Exposure level Cases Controls OR A1 B1 OR1 High A2 OR2 B2 Medium A3 B3 OR3 Low Reference C D Not exposed

23. Relation of Hepato cellular Adenoma to duration of oralcontraceptive use in 79 cases and 220 controls Months of OC use Cases Controls Odds ratio 0-12 7 121 Ref. 13-36 11 49 3.9 37-60 20 23 15.0 61-84 21 20 18.1 >= 85 20 7 49.7 Total 79 220 Source: Rooks & col. 1979

24. Advantages and disadvantages of case-control studies Advantages Suitable for rare diseases Can explore several exposures Low cost Rapid Can cope with long latency Small sample size No ethical problems Disadvantages Cannot calculate the risk Not suitable for rare exposures Temporal relationship difficult to establish Subject to bias Selection of controls Recall bias …

25. Conclusions • Cohort studies are better but harder to carry out and provide true measure of risk • Case-control studies are rapid and easy to carry out, but only provide estimates of risks • Prefer cohort to case-control when feasible • In field epidemiology, case-control studies are more frequently used