Simultaneous Descriptors of Research Design

Many students are not aware that research design can be simultaneously described in many different ways; for example, a drug trial may be described as being prospective, longitudinal, randomized, double-blind, and placebo-controlled, all at the same time. This article provides examples to explain how studies can be simultaneously prospective and cross-sectional, prospective and longitudinal, retrospective and cross-sectional, and retrospective and longitudinal. The term prospective indicates that the study data are newly collected, whereas the term retrospective indicates that the study data already exist in records and merely need to be extracted for study. The term cross-sectional indicates that the study subjects are studied on a single occasion; that is, at a single point in time. The term longitudinal indicates that the study subjects are followed up and that there is almost always more than one time point at which the subjects are assessed. This article also describes unusual designs, such as cross-sectional randomized controlled trials and retrospective studies with prospective data ascertainment.