In contemporary policy discourses, data are presented as key assets for improving health-care quality: policymakers want health care to become ‘data driven’. In this article, we focus on a particular example of this ambition, namely a new Danish national quality development program for general practitioners (GPs) where doctors are placed in so-called ‘clusters’. In these clusters, GPs are obliged to assess their own and colleagues’ clinical quality with data derived from their own clinics—using comparisons, averages and benchmarks. Based on semi-structured interviews with Danish GPs and drawing on Science and Technology Studies, we explore how GPs understand these data, and what makes them trust—or question—a data analysis. The GPs describe how they change clinical practices based on these discussions of data. So, when and how do data for quality assurance come to influence their perceptions of quality? By exploring these issues, we carve out a role for a sociological engagement with evidence in everyday medical practices. In conclusion, we suggest a need to move from the aim of being data driven to one of being data informed.