Marie Gaille1, Ruth Horn2. 1. Laboratoire SPHERE (UMR 7219), CNRS, University of Paris Diderot, bâtiment Condorcet, case 7093, 5 rue Thomas Mann, 75205, Paris Cedex 13, France. 2. The Ethox Centre, Nuffield Department of Population Health, University of Oxford, Old Road Campus, Oxford, OX3 7LF, UK. ruth.horn@ethox.ox.ac.uk.