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Statistical analyses of ordinal outcomes in randomised controlled trials: protocol for a scoping review

Randomised controlled trials aim to assess the impact of one (or more) health interventions relative to other standard interventions. RCTs sometimes use an ordinal outcome, which is an endpoint that comprises of multiple, monotonically ordered categories that are not necessarily separated by a quantifiable distance. Ordinal outcomes are appealing in clinical settings as disease states can represent meaningful categories that may be of clinical importance. They can also retain information and increase statistical power compared to dichotomised outcomes. Target parameters for ordinal outcomes in RCTs may vary depending on the nature of the research question, the modelling assumptions, and the expertise of the data analyst. The aim of this scoping review is to systematically describe the use of ordinal outcomes in contemporary RCTs. Specifically, we aim to (i) identify which target parameters are of interest in trials that use an ordinal outcome; (ii) describe how ordinal outcomes are analysed in RCTs to estimate a treatment effect; and (iii) describe whether RCTs that use an ordinal outcome adequately report key methodological aspects specific to the analysis of the outcome. Results from this review will outline the current state of practice of the use of ordinal outcomes in RCTs. Ways to improve the analysis and reporting of ordinal outcomes in RCTs will be discussed. We will review RCTs that are published in the top four medical journals (BMJ, NEJM, The Lancet and JAMA) between 1 January 2012 and 31 July 2022 that use an ordinal outcome. The review will be conducted using PubMed. Our review will adhere to guidelines for scoping reviews as described in the PRISMA-ScR checklist. The study characteristics and design, including the target parameter(s) and statistical methods will be extracted from eligible studies. The data will be summarised using descriptive statistics.