Prospective versus retrospective: a simple matter of timing?

<ns3:p>Background The terms “prospective” and “retrospective” are commonly used to describe the timing of a study. However, there is much confusion surrounding the use of these terms. In this paper we describe two common definitions used for these terms and explore whether the application of these different definitions may partly explain the confusion surrounding the use of the terms “prospective” and “retrospective”. Methods The most used definitions explored in this paper are a) time of recording of exposure status with respect to the occurrence of an outcome, and b) timing of the accumulation of person-time with respect to the study’s conduct. Subsequently, we apply these definitions to various real-world scenarios. Results Using these definitions, we show that there are specific scenarios in which these definitions may lead to concordant conclusions. However, more worryingly, we also present scenarios in which the two definitions lead to discordant conclusions, thereby partly explaining the confusion caused by these terms. Finally, we propose to use the terms “prospective” and “retrospective” to describe an aspect of a study not covered by other epidemiological concepts (e.g. bias, study design), namely the chronological relationship between the implementation of the eligibility criteria (which may include obtaining informed consent) and the accumulation of person-time. Conclusion In this paper, we show that different definitions used may partly explain the confusion surrounding the term prospective/retrospective. A consistent application of a clear definition will in the future reduce this confusion in the medical literature.</ns3:p>