As the volume of biomedical information escalates and its uses diversify, systematic reviews and meta-analyses – the compilation, selection and statistical analysis of pooled results from similar studies – are becoming an increasingly accepted method in the evaluation of healthcare technologies and interventions. We thus observe a proliferation of laboratories conducting this type of research. How is knowledge constructed in systematic reviews and meta-analysis in healthcare? Drawing on ethnographic data collected during 18 months of fieldwork in a research centre devoted to the development of evidence-based clinical-practice guidelines and systematic reviews, the paper argues that knowledge construction in secondary research in healthcare is structured upon a parallel process of disentanglement and qualification of data. In disentanglement, knowledge practices attempt to extricate data from the milieus in which they are commonly found (databases, texts, other research centres, etc.). In qualification, the focus of activities is on endowing data with new qualities – such as precision, unbiasness or ‘fairness’– through the use of templates, graphical platforms and techno-political debates. The accomplishment of these two processes is fundamental to establishing the persuasive power that meta-analyses appear to have in contemporary healthcare politics.