Prospective Versus Cross-Sectional Study Design – Wordplay of Timing of Study Conduct and Measurement

This is in reference to the article, 'Genital psoriasis: A prospective, observational, single-centre study on prevalence, clinical features, risk factors, and its impact on quality of life and sexual health'.[1] The authors have performed a meritorious job of assessing the prevalence and enlisting the clinical features and risk factors of genital psoriasis and comment on its impact on quality of life and sexual health. However, I have one but significant observation in the present study. The title and methodology of the study mention that it is a 'prospective cross-sectional study'. The study design cannot be both prospective and cross-sectional. Although the present study was conducted over a span of 6 months, it does not qualify to be called as a prospective study, as in a prospective cohort study, the participants are selected based on the exposure status of the individual.[2] In a cross-sectional study, the investigator measures the outcome and exposure of the study participants at the same time. Unlike in cohort studies, where participants are selected based on their exposure status, the participants in a cross-sectional study are just selected based on the inclusion and exclusion criteria set for the study.[3] Observational studies can be broadly classified as directional (prospective cohort and retrospective case-control) or snap-shot (cross-sectional) studies. The terms 'prospective' and 'retrospective' refer to the timing of the research in relation to the development of the outcome. In prospective studies, the outcome (and sometimes even the exposure or intervention) has not occurred when the study starts, and participants are followed up over a period of time to determine the occurrence of outcomes.[4] These studies are used to estimate the cumulative incidence and incidence rate, and the authors have derived prevalence rates from the current study, which can be calculated in cross-sectional studies only. Here, it is imperative to take a similar example from a classic prospective Danish cohort study on psoriasis and depression conducted by Jensen et.al. in 2015.[5] It will be worthwhile to read the design of this study, which will help the readers understand the practical aspects of conducting cohort studies. In the present study, as such, the exposure and outcome have been measured at the same time, which makes it a cross-sectional study design.[4] As there is no temporality/directionality in a snap-shot observational study, the term 'prospective study' should be omitted for clarity and correctness of the literature. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.