BMC Public Health 2009, 9:140 http://www.biomedcentral.com/1471-2458/9/140 ity losses during work (presenteeism). These aspects of the authors provided comments on the drafts of the manu- study increase internal validity. The external validity of the results of the cost-effectiveness analysis is improved by the choice of a societal perspective of the analysis as we can disaggregate the calculated cost- effectiveness from the societal perspective to other levels, such as cost-effectiveness for employers [33]. Moreover, external validity is high as this study can be considered a pragmatic randomized trial, which means the RTW-E pro- gramme is evaluated in the context and reality of circum- stances in which OPs operate daily. To stimulate compliance with the RTW-E programme, we offered OPs in the intervention group a two-day training before inclusion started and three follow-up sessions dur- ing the inclusion period. The follow-up sessions offered OPs the opportunity to discuss difficulties and possible solutions in applying the interventions of the RTW-E pro- gramme with colleagues. A higher compliance with the RTW-E programme increases the level to which differ- ences between groups could be attributed to the RTW-E programme. Weaknesses In this study, we selected patients on sick leave via OPs, and OPs were randomized instead of patients. Although this is an efficient way to include patients in the occupa- tional health field, we may have introduced selection bias by doing this. Furthermore, OPs were not blinded to the RTW-E programme as they were trained in the new pro- gramme. This could also have introduced selection bias. These aspects of selection bias could possibly exaggerate the effect size of the study and thereby decrease internal validity. The OPs could have had a tendency to invite favourable patients if they reasoned that the RTW-E pro- gramme was appropriate for them, instead of inviting and including every patient with a CMD [54]. Although we monitored compliance with the RTW-E pro- gramme and judged the quality of the filled-in homework assignment forms, the counselling sessions remained mainly a black box, as we did not audio- or videotape treatment sessions and based our judgement of compli- ance on a list of treatment criteria. Competing interests The authors declare that they have no competing interests. Authors' contributions JvdK conceived the rationale of the intervention. EN and KN developed the intervention. KN, JvdK and FvD partic- ipated in the design and co-ordination of the trial. EN car- ried out data collection and wrote the manuscript. All script and have read and approved the final version. Acknowledgements We have obtained a grant from the STECR Aladdin programme. 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