' Translation Invariance in Data Envelopment Analysis,' Kerstens, Kristiaan & Van de Woestyne, Ignace, 2009.Ģ009/04, Hogeschool-Universiteit Brussel, Faculteit Economie en Management.' Negative data in DEA: a simple proportional distance function approach,' ' Negative Data in DEA: A Simple Proportional Distance Function Approach,' Kristiaan Kerstens & Ignace Van de Woestyne, 2009.