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Quantitative study designs

Cohort Study

Did you know that the majority of people will develop a diagnosable mental illness whilst only a minority will experience enduring mental health?  Or that groups of people at risk of having high blood pressure and other related health issues by the age of 38 can be identified in childhood?  Or that a poor credit rating can be indicative of a person’s health status?

These findings (and more) have come out of a large cohort study started in 1972 by researchers at the University of Otago in New Zealand.  This study is known as The Dunedin Study and it has followed the lives of 1037 babies born between 1 April 1972 and 31 March 1973 since their birth. The study is now in its fifth decade and has produced over 1200 publications and reports, many of which have helped inform policy makers in New Zealand and overseas.

In Introduction to Study Designs, we learnt that there are many different study design types and that these are divided into two categories:  Experimental and Observational. Cohort Studies are a type of observational study. 

What is a Cohort Study design?

  • Cohort studies are longitudinal, observational studies, which investigate predictive risk factors and health outcomes. 
  • They differ from clinical trials, in that no intervention, treatment, or exposure is administered to the participants. The factors of interest to researchers already exist in the study group under investigation.
  • Study participants are observed over a period of time. The incidence of disease in the exposed group is compared with the incidence of disease in the unexposed group.
  • Because of the observational nature of cohort studies they can only find correlation between a risk factor and disease rather than the cause. 

Cohort studies are useful if:

  • There is a persuasive hypothesis linking an exposure to an outcome.
  • The time between exposure and outcome is not too long (adding to the study costs and increasing the risk of participant attrition).
  • The outcome is not too rare.

The stages of a Cohort Study

  • A cohort study starts with the selection of a group of participants (known as a ‘cohort’) sourced from the same population, who must be free of the outcome under investigation but have the potential to develop that outcome.
  • The participants must be identical, having common characteristics except for their exposure status.
  • The participants are divided into two groups – the first group is the ‘exposure’ group, the second group is free of the exposure. 

Types of Cohort Studies

There are two types of cohort studies:  Prospective and Retrospective.

  • The two groups of cohorts (exposed and un-exposed) are followed prospectively over time to track the development of new disease.
  • Example:  In a prospective cohort study researchers compared four different groups of women (two at-risk groups, two low-risk groups) to investigate which groups were more likely to develop PTSD symptoms after a birthing event.
  • Cohorts are defined from a previous point in time, and are not followed up in the future.
  • Information or data is collected from past clinical records and the outcome of interest is investigated.
  • Useful for tracking the progress of a disease with a long latency period.
  • Example:  In a retrospective cohort study researchers used previously collected data to investigate whether there was an association between birth experience and subsequent maternal care-giving attitudes and behaviour over a 12-month period


How Cohort Studies are carried out

Adapted from: Cohort Studies: A brief overview by Terry Shaneyfelt [video]

Which clinical questions does this study design best answer?

Question Type Study Example
Risks What risk factors predict disease? This cohort study looks at dietary and lifestyle risk factors and investigates how they might contribute to hypertension in women.
Aetiology What factors cause these outcomes? This cohort study looks at factors in early life that may predict the occurrence of adolescent suicide.
Prognosis What happens with this disease over time? This cohort study examines the instances of recovery from a first-time episode of psychosis.
Diagnosis If the test is positive, what happens to the patient? This cohort study examines recently released adults from prison who have been diagnosed with both a mental illness and substance use disorder and investigates what happens to them following their diagnosis.

What are the advantages and disadvantages to consider when using a Cohort Study?

  • The only observational study design that directly investigates risk of disease and the factors contributing to it.
  • Ethically safe.
  • Multiple outcomes can be measured.
  • They are good for rare types of exposures, e.g. an exposure to a chemical spill in a factory.
  • Not appropriate for rare diseases or those that take a long time to develop e.g. mesothelioma.
  • Not appropriate for studying multiple exposures.
  • Can be costly and time consuming.

What does a strong Cohort Study look like?

  • The aim of the study is clearly stated.
  • It is clear how the sample population was sourced, including inclusion and exclusion criteria, with justification provided for the sample size.  The sample group accurately reflects the population from which it is drawn.
  • Loss of participants to follow up are stated and explanations provided.
  • The control group is clearly described, including the selection methodology, whether they were from the same sample population, whether randomised or matched to minimise bias and confounding.
  • It is clearly stated whether the study was blinded or not, i.e. whether the investigators were aware of how the subject and control groups were allocated.
  • The methodology was rigorously adhered to.
  • Involves the use of valid measurements (recognised by peers) as well as appropriate statistical tests.
  • The conclusions are logically drawn from the results – the study demonstrates what it says it has demonstrated.
  • Includes a clear description of the data, including accessibility and availability.

What are the pitfalls to look for?

  • Confounding factors within the sample groups may be difficult to identify and control for, thus influencing the results.
  • Participants may move between exposure/non-exposure categories or not properly comply with methodology requirements.
  • Being in the study may influence participants’ behaviour.
  • Too many participants may drop out, thus rendering the results invalid.

Critical appraisal tools

To assist with the critical appraisal of a cohort study here are some useful tools that can be applied.

Critical appraisal checklist for cohort studies (JBI)

CASP appraisal checklist for cohort studies

Real World Examples

Bell, A.F., Rubin, L.H., Davis, J.M., Golding, J., Adejumo, O.A. & Carter, C.S. (2018). The birth experience and subsequent maternal caregiving attitudes and behavior: A birth cohort study. Archives of Women’s Mental Health.

Dykxhoorn, J., Hatcher, S., Roy-Gagnon, M.H., & Colman, I. (2017). Early life predictors of adolescent suicidal thoughts and adverse outcomes in two population-based cohort studies. PLoS ONE, 12(8).

Feeley, N., Hayton, B., Gold, I. & Zelkowitz, P. (2017). A comparative prospective cohort study of women following childbirth: Mothers of low birthweight infants at risk for elevated PTSD symptoms. Journal of Psychosomatic Research, 101, 24–30.

Forman, J.P., Stampfer, M.J. & Curhan, G.C. (2009). Diet and lifestyle risk factors associated with incident hypertension in women. JAMA: Journal of the American Medical Association, 302(4), 401–411.

Suarez, E. (2002). Prognosis and outcome of first-episode psychoses in Hawai’i: Results of the 15-year follow-up of the Honolulu cohort of the WHO international study of schizophrenia. ProQuest Information & Learning, Dissertation Abstracts International: Section B: The Sciences and Engineering, 63(3-B), 1577.

Young, J.T., Heffernan, E., Borschmann, R., Ogloff, J.R.P., Spittal, M.J., Kouyoumdjian, F.G., Preen, D.B., Butler, A., Brophy, L., Crilly, J. & Kinner, S.A. (2018). Dual diagnosis of mental illness and substance use disorder and injury in adults recently released from prison: a prospective cohort study. The Lancet. Public Health, 3(5), e237–e248.

References and Further Reading

Greenhalgh, T. (2014). How to Read a Paper : The Basics of Evidence-Based Medicine, John Wiley & Sons, Incorporated, Somerset, United Kingdom.

Hoffmann, T. a., Bennett, S. P., & Mar, C. D. (2017). Evidence-Based Practice Across the Health Professions (Third edition. ed.): Elsevier.

Song, J.W. & Chung, K.C. (2010). Observational studies: cohort and case-control studies. Plastic and Reconstructive Surgery, 126(6), 2234-42.

Mann, C.J. (2003). Observational research methods. Research design II: cohort, cross sectional, and case-control studies. Emergency Medicine Journal, 20(1), 54-60.