What: A descriptive report on a group of patients with the same disease or exposure.
Use: Good for generating hypotheses.
Limits: No control group; cannot establish association.
Ecological Study
What: Compares disease frequencies in different populations.
Use: Hypothesis generation using population-level data.
Limits: Prone to ecological fallacy (cannot apply group findings to individuals).
Cross-Sectional Study
What: “Snapshot in time” that measures exposure and outcome simultaneously.
Key Question: What is the prevalence?
Limits: Cannot determine temporality (which came first) or causality.
Case-Control Study
What: Starts with diseased (cases) vs. non-diseased (controls) people and looks back retrospectively for exposure.
Key Question: What happened? (associating past exposures with current disease).
Use: Ideal for rare diseases.
Measure: Odds Ratio (OR).
Limits: High risk of recall bias; cannot calculate incidence.
Cohort Study
What: Follows a group (cohort) based on exposure status over time to see who develops the disease. Can be prospective or retrospective.
Key Question: What will happen? (effect of exposure on outcome).
Use: Measures incidence and establishes temporality.
Measure: Relative Risk (RR).
Limits: Inefficient for rare diseases; costly and time-consuming if prospective.
Twin Concordance & Adoption Studies
What: Studies used to disentangle genetic (“nature”) vs. environmental (“nurture”) influences.
Twin Study: Compares trait frequency in identical (monozygotic) vs. fraternal (dizygotic) twins. Higher concordance in identical twins suggests a strong genetic component.
Adoption Study: Compares traits in adopted children to their biological vs. adoptive parents.
