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Epidemiology Kept Simple. Ch 11: Observational Studies. Observational Designs . Cross-sectional : Sample population, no follow-up of individuals  compare disease experience of exposure groups (§11.2 and §11.3)

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Epidemiology Kept Simple

Ch 11: Observational Studies

Ch 11: Observational Designs

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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

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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

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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

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Example: Ecological Data

r = 0.74

Ch 11: Observational Designs

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Example: X-Sectional Survey SES & Mental Disorders

Ch 11: Observational Designs

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§11.4 Cohort Studies

  • Recruit cohort

  • Classify individual as exposed or non-exposed

  • Follow exposed and non-exposed sub-cohorts to determine incidence






RR or RD

RR or RD

RR or RD

RR or RD




Ch 11: Observational Designs

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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

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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

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Case-Control Studies

  • Identify population cases

  • Randomly select non-cases (“controls”)

  • Compare exposure histories in cases & controls

All cases

Exposure histories


Odds Ratio

Sample non-cases

Exposure histories

Ch 11: Observational Designs

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Cross-tabulate disease and exposure status

of cases and controls


Ch 11: Observational Designs

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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

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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

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Longitudinal Designs

Ch 11: Observational Designs

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Longitudinal Designs

Ch 11: Observational Designs

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End of HS 261 Presentation

Ch 11: Observational Designs