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Lecture 7
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  1. Lecture 7 Objective 18. Describe the elements of design of observational studies: case‑control studies (retrospective studies). Discuss the advantages and disadvantages of this design.

  2. Retrospective Case Control Studies • Case-control studies select cases (with disease) and controls (without disease of interest) • sometimes matched on potentially confounding variables

  3. Retrospective Case Control Studies • presence or absence of the risk factor is determined for each individual. • This is the most commonly used design to study etiologic risk factors

  4. Lens Type and Ulcerative Kerititis

  5. IUD use and primary tubal infertility

  6. Physical Activity and MI

  7. Selection of Subjects • based on outcome (disease) status

  8. Assignment of Exposure • exposure determined by genetics, environment or behavior of subject

  9. Temporal Relationship of Exposure • Exposure and onset of disease precedes investigator followup __________________________ t | | | E O I

  10. Measurement of Risk and Association • Risk: Cannot be estimated • Association: Odds Ratio (OR) as an estimate of the relative risk (RR)

  11. Validity • Confounding Bias: may be difficult to get information on potential confounders because of retrospective nature of exposure; matching on potential confounders will not eliminate confounding

  12. Validity • Information Bias: may reduce possibility by "blinding" investigator to outcome when determining exposure status; objective criteria for identifying exposure may also reduce possibility of bias

  13. Validity • Selection Bias: as exposure and outcome have occurred prior to selection, the potential exists to use knowledge of exposure status in selecting subjects

  14. Advantages • best method for studying rare outcomes • normally less expensive than prospective study • can be used to study outcomes with long latent periods; • quickly done

  15. Limitations • must use retrospective information on exposure and confounders • cannot estimate incidence • problems of validity shared with those of retrospective studies

  16. GUIDE FOR EVALUATING A CASE-CONTROL STUDY • Are cases and controls clearly defined? Were these criteria fulfilled in this study? • Were cases and controls comparable? • Do cases and controls come from the same population? Did cases and controls enter the study similarly? Did authors adjust for any differences?

  17. GUIDE FOR EVALUATING A CASE-CONTROL STUDY • Was exposure status clearly defined and consistently obtained? • What was the response rate for cases? controls? • Were cases and controls blinded as to the study hypothesis? • Was the study size sufficiently large to justify the author's conclusions?

  18. Lecture 7 Objective 19. Describe the elements of design of observational studies: cross‑sectional studies. Discuss the advantages and disadvantages of this design.

  19. Crosssectional (Prevalence) Studies • Relate the disease outcome and exposure status at the same time. • These studies are important in establishing the current health status of the population in diseases of short duration.

  20. Cholera and Water Source

  21. Cough and boys smoking

  22. Selection of Subjects • from total population

  23. Assignment of Exposure • exposure determined by genetics, environment or behavior of subject

  24. Temporal Relationship • Investigator (I), Exposure (E) and Outcome (O) are coincident E |O _____________________ t | I

  25. Measurement of Association • Risk: Prevalence (P) • Association: Prevalence Ratio (PR) or Prevalence Difference (PD)

  26. Validity • Confounding Bias: may be difficult to get information on potential confounder because of retrospective nature of exposure

  27. Validity • Information Bias: eliminate through "blinding", although it may be difficult as exposure and outcome are determined simultaneously; reduce possibility with objective diagnostic criteria

  28. Validity • Selection Bias: because outcome has occurred prior to selection, potential exists for knowledge of outcome status to influence selection of subjects

  29. Advantages • done quickly; no need for follow‑up • can be used to study diseases with long latent periods • good for hypothesis generation

  30. Limitations • uses prevalence to measure risk • not useful for studying rare diseases • not useful for studying rare exposures • must use retrospective information on exposure and confounders • problems of validity shared with those of retrospective studies