Epidemiology Kept Simple

1. Epidemiology Kept Simple Ch 11: Observational Studies Ch 11: Observational Designs

2. Observational Designs • Cross-sectional: Sample population, no follow-up of individuals  compare disease experience of exposure groups (§11.2 and §11.3) • Cohort: closed population with individual follow-up over time  compare disease experience of exposure groups (§11.4) • Case-control: all cases and a sample of non-cases from population  compare exposure experience (§11.5) Ch 11: Observational Designs

3. Cross-Sectional Designs • Recall distinction between longitudinal and cross-sectional observations • Recall the distinction between individual and aggregate units of observation • Cross-sectional design with aggregate unit of observation≡ ecological design • Cross-sectional data with individual units ≡ cross-sectional survey Ch 11: Observational Designs

4. Example: Ecological Data Unit of observation = geographic region Exposure = Cig1930 = cigarettes per capita, 1930 Disease = Mortal = lung cancer mortality per 100,000 p-yrs, 1950 Ch 11: Observational Designs

5. Example: Ecological Data r = 0.74 Ch 11: Observational Designs

6. Example: X-Sectional Survey SES & Mental Disorders Ch 11: Observational Designs

7. §11.4 Cohort Studies • Recruit cohort • Classify individual as exposed or non-exposed • Follow exposed and non-exposed sub-cohorts to determine incidence exposed sub-cohort Incidence1 Closed population RR or RD RR or RD RR or RD RR or RD non-exposed sub-cohort Incidence0 Ch 11: Observational Designs

8. 50% heavy smokers survived to 70 British Doctors Cohort 80% of nonsmoker survived to age 70 Source: Doll, R., Peto, R., Wheatley, K., Gray, R., & Sutherland, I. (1994). Mortality in relation to smoking: 40 years' observations on male British doctors. British Medical Journal, 309(6959), 901-911. Ch 11: Observational Designs

9. Example: Historical Cohort • Historical info on exposure to aniline dyes (from work records) were used to compile exposed and non-exposed worker cohorts • Retrospective data from death certificates on bladder cancer occurrence • Result: bladder cancer occurrence was 100 times as frequent in aniline-exposed cohort • Figure shows induction time between exposure onset and bladder CA occurrence Ch 11: Observational Designs

10. Case-Control Studies • Identify population cases • Randomly select non-cases (“controls”) • Compare exposure histories in cases & controls All cases Exposure histories Population Odds Ratio Sample non-cases Exposure histories Ch 11: Observational Designs

11. Case-Control Cross-tabulate disease and exposure status of cases and controls Calculate: Ch 11: Observational Designs

12. Interpretation of the Odds Ratio • When the disease is rare, interpret the OR as if it were an RR • The illustrative OR of 9.3 suggests that tampon users had 9.3 times the risk as non-tampon users • [The suspected brand of tampon has since been removed from the market] Ch 11: Observational Designs

13. Multiple Levels of ExposureHistorical Example: (Wynder & Graham, 1950, p. 212) Exposure may be measured at various levels. In this historical example, smoking is classified into 6 levels. To analyze the table, break-up it up into five separate 2-by-2 tables with each table referencing the nonexposed group as follows: Ch 11: Observational Designs

14. Longitudinal Designs Ch 11: Observational Designs

15. Longitudinal Designs Ch 11: Observational Designs

16. End of HS 261 Presentation Ch 11: Observational Designs