The authors argue for a scientifically rigorous, contextually valid, practically relevant, and stakeholder-inclusive Platinum Standard for evidence-based assessment to improve public administration and third-party governance in the twenty-first century. The Platinum Standard would encompass two contending gold standards. The first gold standard, based on experimental methods, counterfactuals, and average causal effects, is better known and more institutionalized in practice. The second gold standard, based on case studies, comparative methods, triangulation, and causal mechanisms, is less known. The Platinum Standard incorporates both sets of standards and provides a framework for integrating them in practice. It recognizes the wide array of goals and methodologies that are appropriate for assessing the performance of public administration and third-party governance initiatives in a dynamic and globalizing world. The two gold standards currently compete for prominence in the field of evidence-based assessment. By 2020, these gold standards should be part of a more inclusive Platinum Standard.