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220905P - STUDY DESIGN 3: CASE-CONTROL STUDY

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Presented at a Webinar on Research Methodology in Health Sciences at Northern Area Armed Forces Hospital (NAAFH) on 5th September 2022. By Professor Omar Hasan Kasule Sr. MB ChB (MUK). MPH (Harvard), DrPH (Harvard) Professor of Epidemiology and Bioethics

 

LEARNING OBJECTIVES:

  • Definition and types of case-control studies.
  • Design, strengths, and weaknesses of case-control studies.
  • Selection of cases and controls.


KEYWORDS AND TERMS:

  • Case Selection.
  • Case-control.
  • Case-only.
  • Exposure odds ratio.
  • Population base.


BASICS:

  • The case-control study is popular because of its low cost, rapid results, and flexibility.
  • It uses a small number of subjects.
  • It is used for disease (rare and non-rare) as well as non-disease situations.
  • A case-control study can be exploratory or definitive.
  • ‘Cases’ are all diseased individuals in the population and ‘Controls’ are a random sample of disease-free individuals in the same base population.


CASE SELECTION:

  • The source population for cases and controls must be the same.
  • Cases are sourced from clinical records, hospital discharge records, disease registries, data from surveillance programs, employment records, and death certificates.
  • Cases are either all cases of a disease or a sample thereof.
  • Only incident cases (new cases) are selected.


SOURCE OF EXPOSURE INFORMATION:

  • Interviews,
  • Hospital records, pharmacy records, vital records, disease registry, employment records,
  • Environmental data,
  • Genetic determinants,
  • Biomarker,
  • Physical measurements,
  • Laboratory measurements.


STRENGTHS OF THE CASE CONTROL DESIGN:

  • Computation of the OR as an approximation of the RR.
  • Low cost.
  • Short duration.
  • Convenience for subjects because they are contacted/interviewed only once.


WEAKNESSES OF THE CASE-CONTROL DESIGN:

  • RR is approximated and is not measured.
  • Pr(E+/D+) is computed instead of the more informative Pr(D+/E+).
  • The time sequence between exposure and disease outcome is not clear.
  • Vulnerability to bias (misclassification, selection, and confounding).
  • Inability to study multiple outcomes.
  • Lack of precision in evaluating rare exposures.
  • Inability to validate historical exposure information.
  • Inability to control for relevant confounding factors.


DATA COLLECTION and STATISTICAL PARAMETERS:

The contingency tables: The results of a case-control study are set out in a 2x2 contingency table as follows:

  • The proportion was exposed among the diseased.
  • The proportion was exposed among the non-diseased.
  • The most important statistic is the ODDS RATIO (OR) computed as OR = ad/bc.