![]() ![]() This risk factor may be binary (risk factor present = index group versus not present = reference group), for instance diabetes yes/no. In this way, the effect of exposure on outcome can be expressed as a relative risk. Subsequently, the frequency of the outcome, usually the incidence of disease or death over a certain time span, is measured and related to exposure status. ![]() DesignĪ key characteristic of a cohort study is that at the starting point of the study the subjects are identified and their exposure to a risk factor is assessed. ![]() The aim of this article is to describe several issues related to the design of cohort studies, including their strengths and weaknesses, with special focus on the nomenclature of prospective versus retrospective study designs. In these situations, the cohort study – either prospective or retrospective – often forms a suitable observational study design to reliably answer various research questions. TYPES OF RETROSPECTIVE STUDIES TRIALThis could be averted if the same study were conducted retrospectively.Despite its high ranking in the hierarchy of clinical intervention studies, in nephrology practice the randomized clinical trial (RCT), as discussed in the previous article in this series, is frequently not possible due to practical and ethical constraints. However, we are aware that data collected in retrospective studies might be prone to issues like incomplete records, recall bias and confounding, thereby leading to decreased internal validity of studies. And due to the ambiguity and inconsistent nature of these labels, some even recommend abandoning the labels all together (Wylie C. Some experts are of the opinion that the need to differentiate between the two is minimal, as a retrospective cohort study conducted very well could be superior to an average prospective one in terms of evidence-based hierarchy. Is it really necessary? Does it affect my study results? This leads to the question of how important it is to differentiate the two study types. These disparities in definitions lead to confusion among young epidemiologists as to which study type, they are conducting. Meanwhile, some experts consider any follow up study to be prospective even if historical data is used while they define retrospective studies as case control studies (Last, J.M. In some cases, retrospective studies are defined as the cohort studies whose data collections are from the records or historical data, while prospective studies are when data are collected prospectively (Setia M.S. That is, those who develop the research question before data collection are considered to be conducting a prospective study and it is a retrospective study when developed after data collection (Miettinen, O.S. Some experts, though, define both study types based on the time of development of the hypothesis or research question in relation to data collection. Most epidemiological textbooks define prospective cohort study as a type of cohort study where researchers develop and conduct the study on all subjects before they start developing outcomes while in retrospective cohort study, the researchers go back in time to study exposure history for subjects whom at least some of them have developed the outcomes. ![]()
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