Did you know that most people will develop a diagnosable mental illness or disorder, suggesting that 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.
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.
Watch this video Cohort Studies: A Brief Overview (5:33) to learn more about cohort study design and their strengths and limitations.
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.
There are two types of cohort studies: Prospective and Retrospective.
Prospective |
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Retrospective |
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Question Type | Study Example | |
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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. |
Below are some advantages and disadvantages to consider when using a Cohort study.
Advantages
Disadvantages
To assist with the critical appraisal of a cohort study here are some useful tools that can be applied.