This dissertation observes how disciplines shared data practices. Data practices enable scholars and scientists to transform observations into data that can be systematically collected and analysed. The sharing of these practices between disciplines shows that disciplinary boundaries are permeable, it does not explain how they are maintained. Practices of data collection and analysis from observations were shared between disciplines while at the same time disciplines also enforced certain boundaries. This tension between shared practices and creating boundaries, between the disciplinary and interdisciplinary, is the central theme of this dissertation. Two historical case studies exemplify the tension: the first considers the application of statistical methods developed by Adolphe Quetelet in the discipline of botany and the second discusses how the questionnaire method developed by, amongst others, Jules GilliƩron was adopted in the discipline of linguistics. The separate yet connected research cases are both situated in the nineteenth and early twentieth centuries, mostly in France and Belgium. While both cases focus on different data practices and disciplines, together they illustrate the development of emerging social science disciplines, and demonstrate how international disciplinary congresses are important sites of disciplinary and interdisciplinary interaction. By tracing the sharing of data practices between disciplines this dissertation provides a practice-based perspective on discipline formation. It not only suggests interesting cases, but also provides a historiographical framework to research them. In a time when knowledge is increasingly considered a multi, inter, or even extra-disciplinary product, historical studies into the dynamics of disciplines can be both informative and reflexive.