BMC Public Health 2009, 9:140 http://www.biomedcentral.com/1471-2458/9/140 there is a lack of a golden standard. These two methods give an overestimation and underestimation of the real costs, respectively. In the Osterhaus method, production loss is based on the number of days working with hin- drance because of health complaints and an estimation of the efficiency during those days. In the HLQ method, pro- duction loss is based on the number of hours one needs to catch up with the work one was not able to do because of health complaints. The costs of health care were calculated by multiplying the number of contacts and visits to health care professionals, the number of days receiving ambulant or institutional- ized treatment, and the use of medication, with the prices of health care according to the guidelines for prices in the Netherlands [53]. We did not correct for inflation and did not discount costs as the follow-up period of the study is only one year [33]. All the costs are expressed in euros for the relevant refer- ence year 2006, 2007, or 2008, the year in which the costs were generated. Independent measures We measured job and personal characteristics and work- ing conditions at baseline as independent measures. As job characteristics, we assessed profession, industry, number of contract hours, type of contract, and years employed with the employer. As personal characteristics, we measured age, gender, civil status, and educational level. As working conditions, we assessed work pace and workload, emotional strain, decision latitude, autonomy, social support colleagues and social support supervisor, ruminating, enjoyment of the work, and job insecurity. Furthermore, we measured the expected duration of sick leave, diagnosis, psychological complaints, coping style, self-efficacy of return to work, and work load at baseline, as independent measures. We considered all of these parameters to be potential prognostic factors for the dura- tion of sick leave. Statistical analysis Effect evaluation The time until full return to work is a primary and the time until partial return to work a secondary outcome. These outcomes are based on time-to-event data and therefore a survival analysis is appropriate. An intention-to-treat analysis will be performed by means of multilevel analy- sis, with OPs at the primary hierarchical level and patients at the secondary hierarchical level. If suitable software for multilevel analysis is not available, we will use a regular Cox's multivariate regression analysis to answer the first and second research questions. We will use the 2 ll likeli- hood ratio statistical test for testing differences in the out- come scores between groups. The duration of sick leave will be analysed by multilevel ANOVA. To test differences in the prevalence rates, we will use the Mann Whitney U test. Except for the time until partial return to work secondary outcome scores will be analysed by multilevel MANOVA. Cost-effectiveness evaluation To test statistically the differences in cost-effectiveness between groups, we will use the Net Monetary Benefit (NMB) as a parameter instead of the ICER, as this gives us the opportunity to test differences in cost-effectiveness with a regular statistical test. Therefore, we will re-scale the difference in effect between the two programmes into monetary value by using the threshold willingness-to-pay for a unit of effect, and the difference in costs between the two programmes is subtracted from this value. To test the differences, we will use the non-parametric bias-corrected accelerated bootstrapping test. We will also present a cost- effectiveness plane and an acceptability curve, which show the probability that the RTW-E programme is more cost-effective than the CAU. In order to determine the rel- ative impact of changes in values of different parameters on the cost-effectiveness calculation, we will perform one- way and multi-way sensitivity analyses. Discussion The results of this study will provide input for evidence- based treatment options for workers on sick leave due to common mental complaints. We expected the return-to- work programme based on the principles of exposure in vivo (the RTW-E programme) for workers on sick leave due to common mental complaints to be more effective in enhancing full return to work compared with care as usual (CAU). CAU in this study is a rather stringent control con- dition, as Van der Klink et al. [15] found a reduced dura- tion of sick leave (21 days) of this intervention related to the CAU at that time. Nevertheless, we assumed the RTW- E programme to be robust enough to generate significant and clinical relevant differences in the duration of sick leave between groups. Employing a CAU control does have the benefit of practical relevance. However, such a control group could have reduced the contrast between both groups and may have reduced the potential differ- ence in effect size between groups as a consequence. Strengths and weaknesses Strengths In this field study, we regularly measured outcome – every three months during a year – which enabled the detection of relevant changes in clinical status. We used a diagnostic interview to include or exclude patients, aiming to stand- ardize the diagnostic process. To calculate the cost-effec- tiveness, we incorporated, besides losses due to sick leave and the costs of using health care facilities, also productiv- Page 9 of 11 (page number not for citation purposes)