Dear Guillaume Have you found a solution to this issue with MDM? I have been recommended to stay away from Propensity Score Matching and MDM is one of the alternatives suggested to avoid assuming complete randomization. But this does not sound too encouraging. Best, Jonas _____________________________________________________ Postdoctoral Research Fellow Conservation Science Group Department of Zoology University of Cambridge The David Attenborough Building Pembroke Street, Cambridge, CB2 3QZ Phone: +44 7412 885 112 Danish: +45 2990 5192 Skype: jgeldmann