Evaluating the Generalisability of Trial Results: Introducing a Centre- and Trial-Level Generalisability Index

Background: Few randomised controlled trials (RCTs) recruit centres representatively, which may limit the external validity of trial results. Objective: The aim of this study was to propose a proof-of-concept method of assessing the generalisability of the clinical and cost-effectiveness findings of a given RCT. Methods: We developed a generalisability index (Gix), informed by centre-level characteristics, as a measure of centre and trial representativeness. The centre-level Gix quantifies how representative a centre is in relation to its jurisdiction, e.g. a country or health authority. The trial-level Gix quantifies how representative trial recruitment is in relation to clinical practice in the jurisdiction. Taking a real-world RCT as a case study and assuming trial-wide results to represent ‘true jurisdiction values’, we used simulation methods to recreate 5000 RCTs and investigate the relationship between trial representativeness, reflected by the standardised trial-Gix, and the deviation of simulated trial results from the ‘true values’. Results: The simulation study provides evidence that trial results (odds ratio for the primary outcome and incremental quality-adjusted life-years) were influenced by the representativeness of the sample of recruiting centres. Simulated RCTs with the closest results to the ‘true values’ were those whose recruitment closely mirrored the jurisdiction-wide context. Results appeared robust to six alternative specifications of the Gix. Conclusions: Our findings suggest that an unrepresentative selection of centres limits the external validity of trial results. The Gix may be a valuable tool to help facilitate rational selection of trial centres and ensure the generalisability of results at the jurisdiction level.