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Chapter 8 Case-Control Studies

Chapter 8 Case-Control Studies. Chapter Outline. 8.1 Introduction 8.2 Identification of cases and controls 8.3 Obtaining information on exposure 8.4 Data analysis 8.5 Statistical justifications …. Case-Control Studies. Identify cases in a source population

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Chapter 8 Case-Control Studies

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  1. Chapter 8Case-Control Studies

  2. Chapter Outline 8.1 Introduction 8.2 Identification of cases and controls 8.3 Obtaining information on exposure 8.4 Data analysis 8.5 Statistical justifications …

  3. Case-Control Studies • Identify cases in a source population • Select at random non-cases from the same source population • Compare exposure histories in cases & controls Cases Exposure histories Population Odds Ratio Sample non-cases Exposure histories Note: case-control samples do not permit the calculatino of incidence or prevalence!

  4. Case-Control 2-by-2 Table cross-tabulate exposure & disease status If done properly, the OR from the case control sample is stochastically equivalent to the rate ratio in the underlying source population

  5. Illustrative Example 8.2 • Source population: A Seattle area HMO • 175 histologically confirmed prostate cancer cases • 258 similarly aged men selected at random from source population (controls) • Exposure: prior vasectomy  about equal risk of prostate cancer in vasectomized and non-vasectomized men (“no association”)

  6. Interpretation of case-control ORs • Case-control OR is stochastically equivalent to an RR • Multiple justifications are provided in §8.5 • One such justification is shown on the next slide

  7. Case-Control Odds Ratio = Rate Ratio in Underlying Cohort (Incidence Density Sampling) • The 2-by-2 table should be viewed as a sample from a conceptual prospective cohort • Consider following a cohort: as a case occurs, select at random one or more controls

  8. Identifying Cases • Types of cases • Incident cases onset of disease during study interval (preferred) • Prevalent cases onset of disease before or during study interval • Sources of cases • Population surveillance • Hospitals and clinics • Death certificates • Administrative health records • Pharmacy records • Accurate ascertainment of cases = essential!

  9. Selection of Controls • The objective of the control series is to reflect the relative frequency of exposure in the source population (B1/B0≈ T1/T0) •  controls represent a random sample of the source population • If using hospital-based cases → random sample of hospital’s catchment area • Sources of controls in practice • Population-based sample • Hospitals / clinics • “Friend controls” • Nested in a cohort

  10. Number of Controls per Case • Maximum efficiency with 1 control per case (1:1) • To increase statistical power when the number of cases is limited, select up to 4 controls per case (4:1)

  11. Sources of Exposure Information • Interview • Medical records • Birth & death certificates • Employment records • Environmental records • Biological specimens

  12. The quality of the study depends on the quality of its information (GIGO) Gather information for cases and controls in identical manners (blinded review is best) Well designed data collection forms It should be done simply  Good Information

  13. Data Analysis • Covered • Dichotomous exposure • Multiple levels of exposure • Not covered • Matched pairs • Matched tuples

  14. Multiple Levels of Exposure

  15. Multiple Levels of Exposure

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