The study presented in this thesis was conducted within NIVEL, the Netherlands Institute for Health Services Research in Utrecht (www.nivel.nl), the EMGO Institute for Health and Care Research (www.emgo.nl) and the Department of Public and Occupational Health of the VU University Medical Center in Amsterdam, the Netherlands. NIVEL and EMGO Institute for Health and Care Research participate in the Netherlands School of Primary Care Research (CaRe), which was re‐acknowledged in 2005 by the Royal Netherlands Academy of Arts and Sciences (KNAW). The study was initiated by the Dutch Society of Medical Specialists (Orde van Medisch Specialisten) with financial support from the Ministry of Health, Welfare and Sport. Financial support for the printing of the thesis has been kindly provided by NIVEL and EMGO Institute for Health and Care Research.