5.1. Types of Observational Studies: Cohort Studies

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1. Cohort studies

1.1. Prospective cohort studies


Prospective cohort studies (PCS) observe one or more groups of participants longitudinally over time (often years) to determine the incidence of a specific outcome or various outcomes after an exposure or several exposures (for instance medicines, interventions or risk factors).


In principle prospective cohort studies are characterised in the following way: A cohort of participants is drawn from  source population (the sample should be representative of that population). Participants should be free of the outcome(s) of interest but otherwise have common characteristics and the potential to develop that outcome. Baseline information is collected from all participants, notably their exposure status, using exactly the same data collection methods for all. Cohort groups may be selected on the basis of exposures at baseline, e.g., smokers vs. non-smokers. The first group then is the ‘exposure’ group, the second group is free of the exposure. The participants in the two groups are then followed "longitudinally," i.e., over a period of time, usually for years, and assessed at intervals to determine if and when they develop the outcome(s) of interest (e.g., disease) and whether their exposure status changes. The non-exposed group serves as the comparison group (‘control’) providing an estimate of the baseline or expected amount of the outcome or disease occurrence in the community. Comparison groups can be defined at the beginning or created later using data from the study (e.g., age group, amount of a specific food group consumed). In single-group cohort studies (see box) those participants who do not develop the outcome of interest are used as internal controls. The incidence of the outcome in the exposed group is compared with the incidence of the outcome in the non-exposed group (risk or relative risk). If the incidence is substantially different in the exposed group compared to the non-exposed group, the exposure is likely to be associated with the outcome. Investigators can eventually use the analysis to answer various questions, e.g., about the associations between "risk factors" and disease outcomes. For example, one could identify smokers and non-smokers at baseline and compare their subsequent incidence of developing heart disease. Alternatively, one could group subjects based on their body mass index (BMI) and compare their risk of developing heart disease or cancer.

The following figure shows a schematic of the principal setup of a prospective cohort study.


Figure 1: Principal setup of a prospective cohort study with two participant groups.