Unknown confounders did not bias the treatment effect when improving balance of known confounders in randomized trials.

OBJECTIVE To measure if improving balance in known and observed confounders by propensity score (PS) matching yields different treatment effect estimates in randomized controlled trials (RCTs), thus indirectly measuring the influence of unknown confounders. STUDY DESIGN AND SETTING Analysis of individual patient data of 26 large RCTs and comparison of agreement between PS matched samples and the RCT results on one hand to the agreement between subsamples of RCTs (with sample sizes equal to the sample sizes of the PS matched samples) and RCTs on the other by Bland-Altman-Plots and corresponding intra-class correlation coefficients. RESULTS We included data on 213 outcomes from 37 treatment comparisons with 193,620 patients from 26 trials. Bland-Altman-Plots and intra-class correlation coefficients showed better agreement between PS matched analysis and RCTs than between reduced RCTs and RCTs. CONCLUSION We found no indication for a detrimental influence of unknown confounders in PS matched samples of RCTs.