This paper traces the complexities, contingencies and tensions involved in the creation of a new, 'scientific' assessment in what we call Prefecture A in Japan. We start with a thick, granular description of the complicated and ongoing narrative of a new policy emergence. This descriptive account serves as a precursor to our application of an Actor Network Theory (ANT) approach, particularly the concepts of assemblage and assembling, while at the same time eschewing its depoliticising effect of a flat ontology. Our account details that much work and strategising went into achieving the new assessment but also into holding it in place. This included, strategising inside and across the Prefecture, with the National Ministry, with edu-businesses that had psychometric expertise, with politicians and the board of education and with the OECD and the Director of its Education and Skills Directorate. The analysis illustrates the complex, multi-directional and topological cartographies of power that now work in contemporary education policy processes. As such, we suggest a way to transcend the binary of methodological nationalism and methodological globalism evident in much policy sociology in education work.