Observational Study Designs in Epidemiology

Overview of Study Designs

• Epidemiological Study Designs
  • Case report, case series, cross-sectional studies: Descriptive studies, cannot test hypothesis.
  • Correlational studies: Group-level data, not individual, cannot be applied to individuals.
  • Case control and cohort studies: Analytical observational studies, involve comparison groups.

Case Control Study Design

• Identification
  • Group with outcome of interest: Cases
  • Group without outcome of interest: Controls
  • Compare exposure history between cases and controls.
• Directionality
  • Backward looking: Investigators look back in time for exposure.
• Example
  • Smoking (exposure) and lung cancer (outcome) study by Dr. Richard Dahl & Sir Bradford Hill.

Cohort Study Design

• Identification
  • Group with exposure: Exposed
  • Group without exposure: Unexposed
  • Follow both groups over time to see who develops the outcome.
• Directionality
  • Forward looking: Both time and inquiry move forward.
• Types of Cohort Studies
  • Prospective Cohort Study: Follow forward from present into future.
  • Retrospective Cohort Study: Look back in time, then follow forward.

Cohort Study Examples

• Prospective Study Example: Investigating breastfeeding and diarrhea in infants.
• Retrospective Study Example: Cancer development in X-ray technicians over time.

Comparing Case Control and Cohort Studies

• Choosing a Study Design
  • Factors to consider: Nature and frequency of disease, type of exposure, available resources.
  • Case Control Study: Quick, less resources, suitable for rare diseases.
  • Cohort Study: Suitable for rare exposures, common outcomes, requires follow-up.

Conclusion

• Choice of study design depends on disease nature, exposure, and resource availability.
• Both case control and cohort studies have their unique applications in epidemiology.