Harvey's Garden

Study designs

2 min read

Observational Studies (Investigator Observes Only)

  • Case Series

    • 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.

Experimental Studies (Investigator Assigns Intervention)

  • Randomized Controlled Trial (RCT)
    • What: Gold standard study design. Participants are randomly assigned to a treatment or control group. Blinding reduces bias.
    • Key Question: Is this treatment effective?
    • Use: Establishes causality.
    • Limits: Expensive, may have ethical limitations.

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