Cohort Study: Retrospective and Prospective Design

General Design Framework of a Cohort Study

• A cohort study consists of subjects at risk for a disease whose exposure status is assessed at baseline
• Subjects are followed up over time to detect incident disease
• Elements of a prospective cohort study design include:
  • Selection of sample from population or self-allocated groups
  • Measuring the exposure variable in the sample
  • Ensuring the outcome is not present (none of the participants have the disease)
  • Following up different exposure groups for a period of time
  • Measuring the occurrence of the outcome variable in each group

Exposure Evaluation

• Exposure status can have two levels: exposed group and unexposed group
• Exposure evaluation typically starts with a cross-sectional study to determine baseline exposure status and exclude subjects who are not at risk for the disease

Outcome Evaluation

• A central goal of epidemiology is to identify causes (or exposures) that can explain outcomes such as disease occurrence
• Cohort studies can examine the effect of an exposure on more than one outcome

Types of Cohort Studies

• Retrospective cohort study (historical cohort): exposure and outcome have already occurred at the start of the study
• Prospective cohort study: follows subjects over time to detect incident disease
• Retrospective-prospective study: combines both retrospective and prospective data

Advantages and Strengths of Cohort Studies

• Knowing that predictor variable was present before outcome variable occurred (some evidence of causality)
• Valuable in studying rare exposures
• Can directly measure incidence of a disease outcome
• Can study multiple outcomes of a single exposure
• Relative risk (RR) is a measure of association

Disadvantages and Limitations of Cohort Studies

• Expensive and inefficient for studying rare outcomes
• A large number of subjects is usually needed
• Often need a long follow-up period or a very large population
• Loss to follow-up can affect validity of findings

Comparison with Experimental Studies

• In cohort studies, no intervention, treatment, or exposure is given to participants
• In RCTs, participants are randomly allocated to either receive the treatment/intervention or not
• In cohort studies, there is no randomization, and participants are not allocated to groups

Analytic Framework of Cohort Study

• Two-by-two tables are used to organize data from a cohort study
• Two measures of incidence are risks and rates
• Risks and rates can be further manipulated to provide additional information on the effects of the exposure of interest

Learn about the advantages and limitations of cohort studies, a research design that combines retrospective and prospective data to investigate relationships between variables and disease outcomes.