Tools to Assess the External Validity of Randomized Controlled Trials in Systematic Reviews: a Systematic Review of Measurement Properties

Abstract

Background: Internal and external validity are the most relevant components when critically appraising randomized controlled trials (RCTs) for systematic reviews. However, there is no gold standard to assess external validity. This might be related to the heterogeneity of terminology as well as to unclear evidence of the measurement properties of available tools. The aim of this review was to identify tools to assess the external validity of RCTs in systematic reviews and to evaluate the quality of evidence regarding their measurement properties.Methods: A two-phase systematic literature search was performed in four databases: MEDLINE via PubMed, Scopus, PsycINFO via OVID, and CINAHL via EBSCO. First, tools to assess the external validity of RCTs were identified. Second, studies aiming to investigate the measurement properties of these tools were selected. The measurement properties of each included tool were appraised using an adapted version of the COnsensus based Standards for the selection of health Measurement INstruments (COSMIN) guidelines.Results: 34 publications reporting on the development or validation of 26 included tools were included. For 62% of the included tools, there was no evidence of any measurement property. For the remaining tools, reliability was assessed most frequently. Reliability was judged as “sufficient” for three tools (very low quality of evidence). Content validity was rated as “sufficient” for one tool (moderate quality of evidence).Conclusions: Based on these results, no available tool can be fully recommended to assess the external validity of RCTs in systematic reviews. Several steps are required to overcome the identified difficulties to either adapt and validate available tools or to develop a new one. There is a need for more research for this purpose.Trial registration: Prospective registration at Open Science Framework (OSF): https://doi.org/10.17605/OSF.IO/PTG4D