•The three papers document two large-scale human reliability analysis (HRA) empirical studies.•The two studies compared HRA method predictions against operator performance on nuclear power plant (NPP) simulators.•The studies examined the strengths and weaknesses of HRA methods and the factors contributing to variability in HRA results.•Study achievements, lessons learned, recommendations and insights for improving HRA are presented in the papers. This is the last in a series of three papers documenting two large-scale human reliability analysis (HRA) empirical studies – the International HRA Empirical Study and the US HRA Empirical Study. The goal of the two studies was to develop an empirically-based understanding of the performance, strengths, and weaknesses of HRA methods by comparing HRA method predictions against actual operator performance in simulated accident scenarios on nuclear power plant (NPP) simulators. This paper first addresses areas where there is convergence between the two studies and where differences lie. Then it summarizes the combined insights and conclusions, including key findings on HRA in general through lessons learned about the HRA methods assessed in the studies and specific recommendations for improving guidance, practice and methods. Then it discusses the relevance and usefulness of simulator data for HRA in general. Finally, it presents the key achievements and overall conclusions of the two studies taken together.