Specific collaborative group intervention for patients with medically unexplained symptoms in general practice: a cluster randomized controlled trial - Schäfert, Rainer; Kaufmann, Claudia; Wild, Beate; Schellberg, Dieter; Bölter, Regine; Faber, Rainer; Szecsenyi, Joachim; Herzog, Wolfgang (Psychotherapy and psychosomatics, 2013)