Prospective Study

Various types of study designs have been described, such as cohort studies, case-control studies, crosssectional [Page 1428]surveys, and randomized clinical trials. A central feature of such designs is whether a study was conducted in a retrospective or prospective manner. For example, case-control studies are generally considered retrospective in nature and cohort studies are typically prospective; however, this may not always be the case.

In determining the appropriate study design, many factors are considered and include availability of resources, exposure, outcome/event of interest, sample size, potential confounders and biases, cost, time, and other variables. The general view is that if the phenomena (outcome/event) of interest has already occurred prior to the initiation of the study and various potential confounds may exist that present a challenge to control, the study is regarded retrospective in nature.

Conversely, a study is deemed prospective if the study's subjects or sample group are followed through in time, either randomly or nonrandomly allocated, to assess the development of the outcome/event. Therefore, prospective studies may account for the temporal sequencing of outcomes/events, and control or minimize confounding variables and biases. However, a prospective study design may be more costly and time consuming in comparison to a retrospective study. In addition, obtaining an adequate sample size is often more challenging in prospective than retrospective studies. Thus, retrospective study designs rely on more historical events and prospective study designs represent out-comes/events that manifest concurrency with the timing of the study. In addition, prospective studies are regarded as being “stronger” or more reliable in design and with a higher level of evidence than are retrospective studies.

In lieu of the general views of prospective and retrospective study designs as noted above, classification of studies into such categorizations is, unfortunately, more complex. Various nuances of such study designs may exist. Furthermore, such study designs may often coexist, thereby, studies may utilize both prospective and retrospective components.

Overall, the prospective study decreases the risk of bias or recall in respect to the order of the exposure. A retrospective study can influence the analyses and interpretation of the data parameters and affect the study's validity if one had to rely on information already collected and noted, such in the case of clinical chart reviews. In addition, assessment of whether an outcome/event does not influence the exposure may assist in determining if a study design is prospective.