BMC Public Health 2009, 9:251 http://www.biomedcentral.com/1471-2458/9/251 Consequently, the (cost-)effectiveness is explored in a het- erogeneous group of patients. Measuring heterogeneity of our study population with the SCID-I/P and VKP but not excluding co-morbidity and drawing attention to the group training by announcements to both physicians and patients acumilate this heterogeneity. Patients being referred on their own initiative are probably more moti- vated than patients being referred on their physician's ini- tiative. On the one hand, this enhances the probability of a representative study population, whose heterogeneity will be equally divided between the two conditions by randomization. Furthermore, this heterogeneous study population is real practice, making the study results real- istic estimates of that practice. On the other hand, the group training might be effective by only treating the co- morbidity, like anxiety. Analyzing the predictability of this co-morbidity and other baseline variables on (cost- )effectiveness can solve this limitation. Because the daily practice only allows a waiting list for a short period, the follow-up measurements do not have a control condition. Repeated measurements in the same patients and using a probabilistic model can solve this problem partly. If we show that our group training is feasible in daily prac- tice and (cost-)effective, more patients with UPS could be served, their quality of life could be improved while costs might be decreased. Abbreviations used UPS: Unexplained Physical Symptom(s); DSM-IV-TR: Diagnostic and Statistical Manual of Mental Disorders IV -Text Revision; SCID-I/P: Structured Clinical Interview for DSM-IV Axis I Disorders/Patient edition; VKP: Self-report questionnaire for DSM-IV Axis II personality disorders; IPDE: International Personality Disorder Examination; SF-36: Short Form Health Survey; SF-6D: Short Form 6 Dimensions; SCL-90-R: Symptom Checklist Revised; TiC- P: Trimbos/iMTA Questionnaire for Costs associated with Psychiatric Illness; HLQ: Health and Labour Question- naire. Competing interests The authors declare that they have no competing interests. Authors' contributions LNLZ developed the original idea of the study, wrote the ad verbatim manual for the group training, implemented the study in the daily practice of the real life mental healthcare institution, Riagg Rijnmond, The Netherlands. HJD substantially contributed to the randomization and the statistical methods. 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