Drawing on Kane's argument-based approach to validity and Toulmin's later work on cosmopolitanism and diversity, this paper asks whose validity arguments and evidence are being presented in International Large-Scale Assessments (ILSAs), where and when. With a case study of the OECD's PISA for Development, we demonstrate that validity arguments are assembled, negotiated and transformed by the network of actors. We claim that the challenge of ILSAs is not to establish a single authoritative argument through the displacement of plural interpretations and uses. Instead, the tasks of an argument-based approach should be to create a democratic space in which legitimately diverse arguments and intentions can be recognized, considered, assembled and displayed.