Heterogeneous employment effects of job search programmes : a machine learning approach / Michael C. Knaus (SEW, University of St. Gallen and IZA), Michael Lechner (SEW, University of St. Gallen, CEPR, CESifo, IAB and IZA), Anthony Strittmatter (SEW, University of St. Gallen) ; IZA Institute of Labor Economics. Bonn, Germany : IZA Institute of Labor Economics, August 2017