Core outcome sets are valuable, but methodological evidence can improve robustness.

A core outcome set (COS) is the agreed minimum set of outcomes to be measured in studies regarding a specific topic. A COS is considered to encompass the most relevant outcomes and does not restrict researchers. One should realize that outcomes not included in the COS may actually be important for specific research questions and different study-designs. The COMET handbook (Core Outcome Measures in Effectiveness Trials), used in the current study, describes consensus methodology for COS development. (Williamson et al. Trials 2017, 18(Suppl 3):280). In a nutshell, it is advised to start with a systematic review to identify all possible outcomes; then use the Delphi strategy to converge opinions to consensus; and finally, the prioritized list of outcomes is discussed in a face-to-face consensus meeting in which the final COS is conducted.