Case-Control Studies Overview

Case-Control Studies

• Case-control studies are observational analytic studies
• These studies analyze existing data.
• A case-control study involves selecting individuals with a disease (cases) and a similar group of individuals without the disease (controls).
• Researchers then look back to determine whether the cases were more frequently exposed to a suspected risk factor than the controls.

Objectives

• Describe the major principles of case-control studies.
• Contrast and compare case-control studies to other study designs.
• Evaluate the suitability of case control studies for epidemiological questions.
• Explain the limitations of case-control studies.
• Calculate and interpret the odds ratio (OR) and 95% confidence intervals (CI).

Types of Epidemiological Studies

• Descriptive studies: Describe the occurrence of disease in populations. This includes:
  • Case reports and case series: detailed accounts of individual cases or a group of similar cases.
  • Correlation studies: determine if a relationship exists between two or more phenomena.
  • Cross-sectional studies: assess the prevalence of disease or exposure in a population at one point in time.
• Analytic studies: Determine the cause-and-effect relationship between exposures and outcomes. This includes:
  • Observational studies:
    • Case-control studies: compare exposures in individuals with a disease (cases) to those without the disease (controls).
    • Cohort studies: follow groups of individuals over time to identify the relationships between exposures and disease.
  • Interventional (experimental) studies: investigators actively intervene by manipulating an exposure or treatment.

Conducting a Case-Control Study

• Identify cases: selecting individuals with the disease. This includes strict diagnosis, case definition, and diagnostic criteria. This section also details sources of selecting cases such as hospital-based or population-based studies.
• Identify controls: matching controls to cases to important variables or features, ensuring that the controls also come from the relevant population. Sources of selecting controls also includes hospital-based or a general population or even specific populations.
• Document exposures: record past exposures among cases and controls.
• Calculate odds ratios (OR) and 95% confidence intervals (CI): measure the association between exposure and disease.
• Interpret study results.

Matching in Case-Control Studies

• Matching is matching controls to cases to relevant variables to minimize confounding factors such as age, occupation, race, and sex.
  • 1:1 matching is typical in large case-control studies.
  • Multiple controls are matched to each subject in small studies (1:4).

Criteria for Controls

• Controls must lack the disease that is being studied.
• Controls should represent the population from which the cases arose.
• The selection of controls is independent from exposure.

Measures of Association

• The Odds Ratio (OR) is a measure of association used in case-control studies.
  • It is the ratio of the odds of exposure among cases to the odds of exposure among controls.
  • A value of OR = 1 indicates no association.
  • An OR < 1 suggests a protective effect associated with the exposure.
  • An OR > 1 suggests a positive association with the disease.

Interpreting Odds Ratios and 95% Confidence Intervals

• Odds ratios are subject to random variation.
• A 95% confidence interval (CI) provides a range of plausible values for the true or underlying odds ratio.

Strengths of Case-Control Studies

• Quick and inexpensive—often less time and costly than cohort studies.
• Suitable for diseases with long latent periods (time between exposure and disease).
• Useful for studying rare diseases or exposures where it is difficult to follow sufficient numbers of people over a long time.
• Can investigate multiple exposure factors that impact a single disease.

Limitations of Case-Control Studies

• Inefficient for studying rare exposures.
• Cannot estimate the incidence rate of a disease.
• Difficult to determine the temporal relationship between exposure and disease (exposure may occur after disease onset).
• Prone to bias and confounding such as selection and recall bias.

Case-Control vs. Cohort Studies

• Case-control studies are efficient for rare diseases; cohort studies are more efficient for rare exposures.
• Case-control studies examine outcomes; cohort studies examine exposures.
• Case-control studies are measured using odds ratio; cohort studies are measured by relative risk.

Examples of Case-Control Studies

• Autism and vaccination
• Lung cancer and radon exposure

Types of Bias

• Selection bias: bias in the recruitment of study participants.
• Recall bias: bias from participants' past memories of past exposures.
• Misclassification bias: bias from inaccurate measurements of exposure or disease status.

This quiz covers the fundamentals of case-control studies, an important type of observational analytic research in epidemiology. You will learn the principles, comparisons with other study designs, and how to evaluate their suitability for investigating epidemiological questions. Additionally, it includes calculating and interpreting odds ratios and confidence intervals.