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A prospective cohort study is a longitudinal observational study in which researchers follow a group of individuals (a cohort) forward in time to examine how specific exposures influence the development of health outcomes.

Prospective cohort studies can measure multiple outcomes and establish causality, however, they can be time consuming and costly.

Requirements:

  • Time requirement is dependent on the study objectives, sample size and outcome latency.
    • Short term studies can take up to 5 years.
    • There are long term studies that have taken over 15 years. 
  • The size of your research team depends on the study scale, duration, and funding.
    • A small study (e.g., <500 participants) may require 5–10 people, while a large-scale study (e.g., 10,000+ participants) may need dozens of researchers over time.
  • Cohort studies should adhere to the STROBE Statement.
  • You will need to obtain ethics approval and informed consent.

How to do a prospective cohort study

  1. Develop a hypothesis and aim
  2. Search the literature
    1. You may need assistance from a librarian
  3. Develop the study methodology
    1. Consider target population, recruitment methods, sampling methods, and inclusion/exclusion criteria.
  4. Obtain approval 
  5. Obtain informed consent
  6. Write and register the protocol
  7. Obtain funding
  8. Collect baseline data
  9. Follow participants over time and collect additional data
  10. Analyse data
    1. You may need assistance from a biostatistician
  11. Write the research
  12. Reference your sources
  13. Publish your research

Examples
Kotov, A., et al. (2021). The prospective GermanVasc cohort study. VASA, 50(6), 446–452. 
Wesselink, A. K., et al. (2023). COVID-19 vaccination and menstrual cycle characteristics: A prospective cohort study. Vaccine, 41(29), 4327–4334. 

Recommended resources:

Shaneyfelt, T. (2013). Cohort studies: a brief overview. 

A retrospective cohort study is an observational study that looks back in time to examine the relationship between an exposure and an outcome. Instead of following participants forward in time (like a prospective cohort study), researchers use existing data (e.g., medical records, registries, or employment records) to compare exposed and unexposed groups and assess health outcomes.

Requirements:

  • 1-2 years and 5-12 researchers, depending on size and the complexity of the data.
  • Cohort studies should adhere to the STROBE Statement.
  • You may need to obtain ethics approval and informed consent.

How to do a retrospective cohort study

  1. Develop a hypothesis and aim
  2. Search the literature
    1. You may need assistance from a librarian
  3. Develop the study methodology
    1. Consider target population, data sources, sampling methods, outcomes of interest, and confounding variables.
  4. Obtain approval 
  5. Obtain informed consent
  6. Collect data
  7. Analyse data
    1. You may need assistance from a biostatistician
  8. Write the research
  9. Reference your sources
  10. Publish your research

Examples
Liu, H., et al.  (2021). Establishment of a prognostic model for patients with sepsis based on SOFA: a retrospective cohort study. The Journal of international medical research, 49(9), 3000605211044892. 
Mei, Z., et al. (2022). A nomogram to predict hyperkalemia in patients with hemodialysis: a retrospective cohort study. BMC nephrology, 23(1), 351.

Recommended resources:

Epidemiology Stuff. (2022). Prospective vs Retrospective Cohort Studies.

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