Cohort Studies are observational studies on groups of people with defined characteristics in which outcomes related to particular exposure (or lack thereof) are compared. Cohort Studies are usually indicated in studies where manipulated exposure is considered to be unethical (eg. no group of people should be asked to smoke for the purpose of outcome comparison). Similarly, these are observational studies, thus they lack the opportunity to control or prevent the expected outcome.
Hierarchy of Evidence
Cohort Studies Advantages & Disadvantages
Cohort Studies need to include a control group – a group which is not exposed to the risk factor of interest. Participants are selected based on their exposure status at the start of the study, and exposed and unexposed groups need to be selected from the same population.
Advantages
- exposure to the risk factor of interest is measured prior to disease onset, which reduced bias
- rare exposures can be examined by appropriate selection of study cohorts
- multiple outcomes can be studied for a single type of exposure
- calculates incidence and relative risk of disease in both exposed and unexposed participants over time
Disadvantages
- changes in the participants’ exposure status and diagnostic criteria that may happen over time can affect the individuals’ classification based on exposure and disease status; the researcher should think about what measures may need to be taken if the participants change their patterns throughout the study period
- risk of information bias – outcome may be influenced by information on the participant’s exposure status
- loss of follow-ups may introduce attrition bias, where the characteristics of drop-outs and those completing the study may be significantly different, leading to a reduction in the validity of the study
- expensive and time consuming
Preventing Loss to Follow Up
During the recruitment process, the researcher should obtain all information required so that the participant can be easily contacted. In addition, the researcher should exclude participants that are likely to be lost (eg. a prospective participant may have plans to move to another country).
During the follow-up period, the researcher should maintain regular contact through different means, and possibly provide tokens or gifts to encourage continued participation.
Prospective VS Retrospective Cohort Studies
In Prospective Cohort Studies, participants are identified at the time of exposure. They are followed up over time until outcome occurs.
Advantages: Prospective Cohort Studies are designed with specific data collection methods.
Disadvantages: Such studies entail a long indefinite follow-up period until an outcome occurs. They are susceptible to loss of follow-up, and are usually expensive.
In Retrospective Cohort Studies, the chosen participants would have already been exposed to and subsequently experienced an outcome. Thus, outcome data measured in the past is then reconstructed for analysis.
Advantages: Retrospective Cohort Studies are cheaper and quicker than prospective studies, and make use of past data, which can be accessed immediately.
Disadvantages: Such studies are susceptible to both recall bias and information bias, and may be subjected to incomplete, inaccurate, or inconsistent data due to limited control over data collection.
Cohort Studies Critical Appraisal
Casp Tool
CASP Tool for Cohort Studies Critical Appraisal can be found here.
To view blogpost featuring Cochrane videos on all types of studies please click here.
Types of Statistical Tests Used in Cohort Studies
- Risk Ratio (RR)
- Odds Ratio (OR)
- Confidence Interval (CI)
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Claire is keen about public education on health-related subjects as well as holistic patient-centered care. She is also passionate about spreading awareness on the negative effects that domestic abuse leaves on its victims’ mental, emotional, social and physical wellbeing.
Claire aspires to continue studying following completion of her Nursing Degree, because she truly believes in lifelong education.
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