“…Frölich (u) SIAW, Universität St. Gallen,Bodanstr. 8,9000 St. Gallen, Switzerland e-mail: markus.froelich@unisg.ch 1 See e.g., Black and Smith (2004), , Gerfin and Lechner (2002), Heckman et al (1998a), Frölich et al (2004), Larsson (2003), Lechner (1999Lechner ( , 2002, Sianesi (2004) or Smith and Todd (2005). 2 Hahn (1998) showed this for the case of experimental data, where the propensity score is constant and not affected by the X variables.…”