Dimitris Challoumas

External validity is an important parameter that needs to be considered for decision making in health research, but no widely accepted measurement tool for the assessment of external validity of randomized controlled trials (RCTs) exists. One of the most limiting factors for creating such a tool is probably the substantial heterogeneity and lack of consensus in this field. The objective of this study was to reach consensus on a definition of external validity and on criteria to assess the external validity of RCTs included in systematic reviews. A three‐round online Delphi study was conducted. The development of the Delphi survey was based on findings from a previous systematic review. Potential panelists were identified through a comprehensive web search. Consensus was reached when at least 67% of the panelists agreed to a proposal. Eighty‐four panelists from different countries and various disciplines participated in at least one round of this study. Consensus was reached on the definition of external validity (“External validity is the extent to which results of trials provide an acceptable basis for generalization to other circumstances such as variations in populations, settings, interventions, outcomes, or other relevant contextual factors”), and on 14 criteria to assess the external validity of RCTs in systematic reviews. The results of this Delphi study provide a consensus‐based reference standard for future tool development. Future research should focus on adapting, pilot testing, and validating these criteria to develop measurement tools for the assessment of external validity.