It requires a large number of individuals to be followed up for long periods of time and this can be difficult due to loss to follow-up or withdrawal by the individuals studied. The main disadvantage to prospective cohort studies is the cost.

The study is carried out in three fundamental stages: identification of the individuals, observation of each group over time to evaluate the development of the disease in the groups, and comparison of the risk of onset of the disease between exposed and non-exposed groups. This design provides a different advantage over case-control studies in which exposure and disease are assessed at the same time. Prospective cohort studies are characterised by the selection of the cohort and the measurement of risk factors or exposures before the outcome occurs, thus establishing temporality, an important factor in determining causality. The disadvantages are the weaknesses of observational design, the inefficiency to study rare diseases or those with long periods of latency, high costs, time consuming, and the loss of participants throughout the follow-up which may compromise the validity of the results. In addition, cohort studies are less susceptible to selection bias than case-control studies. Īdvantages of cohort studies include the possibility of examining multiple results from a given exposure, determining disease rates in exposed and unexposed individuals over time, and investigating multiple exposures. In order to determine if the sample studied reflects a real effect of the risk factor in the population, the sample variability of the findings may be evaluated through tests of significance or confidence intervals. If the incidence in the two groups is equal, the value for the RR will be 1, but if the value is greater than 1, this indicates a positive relationship between the risk factor and the outcome. It compares the incidence of the disease in the exposed group with the incidence in the non-exposed group, hence the name relative risk or risk ratio. Relative risk (RR) is the measure of association that is applied for the analysis of the results in cohort studies. Subsequently, the frequency of the outcome, usually the incidence of disease or death over a period of time, is measured and related to exposure status. A fundamental characteristic of the study is that at the starting point, subjects are identified and exposure to particular risk factors is assessed. Because exposure is identified before outcome, cohort studies are considered to provide stronger scientific evidence than other observational studies such as case-control studies. Ĭohort study design is described as ‘observational’ because, unlike clinical studies, there is no intervention. The epidemiological definition of the word currently means:Ī group of people with certain characteristics, followed up in order to determine incidence or mortality by any specific disease, all causes of death or some other outcome.

Frost, a 20th century epidemiologist, was the first to adopt the term in a 1935 publication, when he assessed age-specific and tuberculosis-specific mortality rates. In epidemiology, the term “cohort” is used to define a set of people followed for a certain period of time.

Types of retrospective studies