One-stage and two-stage meta-analysis of individual participant data led to consistent summarized evidence: lessons learned from combining multiple databases

Our study offers further evidence that two-stage approach generates estimates very similar as those from one-stage approach, even in the case of between-DB exposure heterogeneity and when several covariates must be concurrently considered. As current rules limit the free movement of electronic health data, our findings open the door of treating data within the country where they are generated and then to apply conventional techniques for summarizing estimates, which is the two-stage approach for meta-analysis using individual participant data.