BMC Public Health 2009, 9:140 http://www.biomedcentral.com/1471-2458/9/140 intervention during the inclusion period to OPs in the control group. Furthermore, the chance of contamination was small as almost every OP in the study worked at a geo- graphically different worksite. The design we used is presented in Figure 1. The Medical Ethics Committee of the Academic Medical Center and University of Amsterdam approved the study design, research protocol, information brochure, and informed consent. Patients voluntarily participated in the study. A written informed consent was obtained from each patient. Patients were informed about their right to withdraw from the study at any time, without specifica- tion of reasons and with no negative consequences for their treatment. Patients who withdrew received care as usual. The acronym we used for the study is 'WORK UP study'. Sample size and randomization In this study, we wanted to include 60 OPs who would in turn include 200 patients in order to be able to detect a statistically significant difference in sick leave between groups. This sample size was not based on a power analy- sis as software for such an analysis of survival data was not available at the time. In a comparable intervention study of Van der Klink et al. [15], significant differences were found with 33 OPs (17 in the experimental and 16 in the control group) and 192 patients (109 experimental and 83 control group). In that study, each OP included an average of 6 patients (range 1–16). After randomization, we expected the experimental and control groups each to consist of 30 OPs, who were expected to include 3 or 4 patients each. To realize the inclusion of 200 patients, we requested every OP to include 8 patients as we expected a high drop-out rate of OPs e.g. caused by a dynamic labour market. OPs presented themselves for participation in the study. All the presented OPs were randomized to the experimen- tal or control group. We performed a restricted randomi- zation with blocks of four OPs. Every time four OPs presented themselves to researcher EN, these were rand- omized by researcher KN, concealed from researcher EN. After randomization, researcher KN informed EN about Rando- mization FDiegsuigrneo1f the study Design of the study. n o i t n e ’sPO tirevn rgoup l ’s tro up PO cno rgo Start of sick leave n io ts s n lu ie c t In ap n io ts s n lu ie c t In ap Baseline Inclusion 2–8 weeks of sick leave Measurement of compliance with RTW-E Counselling RTW-E programme Counselling as usual (CAU) Follow- up 1 Follow- up 2 Follow- up 3 Follow- up 4 3 months 6 months 9 months 12 months Page 4 of 11 (page number not for citation purposes)