Prospective cohort design

Prospective Cohort Design - The Forward Pass

• Concept: A "forward-looking" observational study. Starts with an exposure and follows subjects over time to observe outcome development. 📌 Mnemonic: Prospective = Progresses forward in time.
• Measures: Calculates incidence and Relative Risk (RR).
  • $RR = \frac{Incidence_{exposed}}{Incidence_{unexposed}}$
• Strengths:
  • Establishes temporal sequence (exposure before outcome).
  • Ideal for studying rare exposures.
  • Minimizes recall bias.
• Weaknesses:
  • Expensive and time-consuming.
  • Inefficient for rare diseases.
  • Prone to attrition bias (loss to follow-up).

⭐ Exam Favorite: The strongest observational design for establishing a temporal relationship between an exposure and an outcome, a key criterion for causality.

Measures of Association - Risk Rundown

• Relative Risk (RR): Compares the risk of an outcome in the exposed group to the unexposed group. It is the primary measure of association in cohort studies.

  • Formula: $RR = \frac{\text{Risk in Exposed}}{\text{Risk in Unexposed}} = \frac{A/(A+B)}{C/(C+D)}$
  • Interpretation:
    • $RR = \textbf{1}$: No association between exposure and outcome.
    • $RR > \textbf{1}$: Exposure increases the risk of the outcome.
    • $RR < \textbf{1}$: Exposure decreases the risk (protective effect).

• Attributable Risk (AR): The absolute difference in risk between the exposed and unexposed groups; the excess risk due to exposure.

  • Formula: $AR = \text{Risk in Exposed} - \text{Risk in Unexposed}$

• Attributable Risk Percent (AR%): The percentage of disease in the exposed group that is attributable to the exposure.

  • Formula: $AR% = \frac{RR-1}{RR} \times 100%$

⭐ Relative risk can only be calculated from cohort studies as it requires incidence data. Case-control studies, which do not follow subjects over time, use the odds ratio as an estimate.

Strengths vs. Weaknesses - Pros & Pitfalls

• Strengths

  • Establishes temporality (exposure precedes outcome).
  • Allows for direct calculation of incidence and relative risk.
  • Excellent for studying rare exposures.
  • Can assess multiple outcomes from a single exposure.
  • Minimizes recall bias compared to retrospective designs.

• Weaknesses

  • Very expensive and time-consuming.
  • Inefficient for rare diseases or outcomes with a long latency period.
  • Requires a large sample size.
  • Loss to follow-up (attrition bias) is a major potential threat to validity.
  • Exposure status may change over the course of the study.

⭐ High-Yield: The key advantage over case-control studies is the ability to establish a clear temporal sequence and directly calculate incidence and relative risk.

High‑Yield Points - ⚡ Biggest Takeaways

• Starts by identifying exposure status and follows subjects forward in time to observe for disease development.
• Directly calculates incidence in both the exposed and unexposed groups.
• The primary measure of association is Relative Risk (RR).
• Establishes a clear temporal relationship between exposure and outcome.
• Ideal for studying rare exposures and multiple outcomes from a single exposure.
• Major limitations: expensive, time-consuming, and high potential for loss to follow-up.