How to Structure a Medical Research Protocol
The essential sections of a protocol or synopsis, and what reviewers look for in each.
Research Methodology
Cross-sectional vs Cohort Studies
How the two most common observational designs differ, what each can tell you, and how to choose between them.
Research Methodology
Both designs are observational — the researcher does not assign exposures — but they answer different questions.
Cross-sectional studies
Exposure and outcome are measured at the same point in time. They are well suited to estimating prevalence and exploring associations. They are relatively quick and inexpensive, which makes them common in postgraduate theses.
The key limitation: because exposure and outcome are measured together, you usually cannot establish which came first.
Cohort studies
Participants are classified by exposure and followed over time to see who develops the outcome. Cohorts can estimate incidence, relative risk and time-to-event outcomes, and establish temporal sequence.
Prospective cohorts need follow-up time and effort to minimise loss to follow-up. Retrospective cohorts use existing records and are faster, but depend on the quality of those records.
Choosing between them
- Want to know how common something is? Cross-sectional.
- Want to know whether an exposure predicts a future outcome? Cohort.
- Limited time for follow-up? A cross-sectional or retrospective cohort design may be more realistic.
Reporting guideline
Both designs are reported using the STROBE statement. Reviewing the STROBE checklist while planning helps you collect everything you will need to report.
This article is for general educational purposes. Always follow your institution’s guidelines and your guide’s advice.