BMC Public Health 2009, 9:251 http://www.biomedcentral.com/1471-2458/9/251 Outcome measurements: clinical evaluation The aim of the clinical evaluation is to investigate the effectiveness of the group training by comparing the improvement of quality of life gained in the training group to the one in the waiting list group. Primary out- come measure is the Mental and Physical summary score of the SF-36. Secondary outcomes are the eight individual scales of the SF-36 and the scales of the SCL-90-R. Outcome measurements: economic evaluation The aim of the economic evaluation is to investigate cost- effectiveness of the group training in terms of cost per Quality Adjusted Life Years (QALYs). QALYs are estimated by converting SF-6D into utilities by means of the prefer- ence-based UK tariff [25]. Indirect costs for employed patients are measured with the TiC-P by the reported duration of sick leave and the production loss without sick leave. The indirect costs of production loss due to sick leave are computed by multiplying the number of sick leave's day with the average net income per worker related to age and gender. By a long-term sick leave, the friction- cost method is applied to assess the productivity loss, using a friction period of 5 months. Sample size calculation To determine the required sample size for measuring dif- ferences in quality of life between the two conditions (group training and waiting list), the sample size is calcu- lated by power analysis. The effect size of cognitive behav- ioral therapy for quality of life is not well known, because in other effect studies the outcome is usually measured in terms of physical symptoms. Therefore, we have to use effect size of cognitive behavioral therapy for physical symptoms as an available estimator for the (shortage of) quality of life. The effect size of cognitive behavioral ther- apy for physical symptoms ranges from .00 to .95 [8], sug- gesting a medium effect for cognitive behavioral therapy for physical symptoms. Assuming this effect also applies for quality of life, the magnitude of the effect size follow- ing Cohen's (1988) [28] is 0.50. With a power of .80 and an alpha of .05 (two-tailed), a sample size of 100 patients (50 in each condition) is required. Statistical analyses The comparability of the patients' baseline-variables between the two conditions (group training and waiting list) is analyzed with the two-tailed t-tests for independent samples for the continuous variables, with the two-tailed Mann-Whitney U-tests for the ordinal variables and with the chi-square tests for the categorical variables. If the patients in the two conditions are not comparable on one or more baseline-variables, those variables will be utilized as covariables in the subsequent analyses. Statistical analyses: clinical evaluation The clinical evaluation is conducted according to the intention-to-treat principle. The effectiveness of the group training for primary and secondary outcome measures is analyzed with mixed modeling (i.c. random regression modeling). Baseline measurements, corresponding to the subsequent outcome measurements, are entered as covari- ables. This method of mixed modeling for repeated meas- urements enables the use of flexible error covariance structures. In addition, the predictive performance of baseline-variables, especially personality variables, on effectiveness can be estimated. Statistical analyses: economic evaluation The economic evaluation is conducted from a societal per- spective, the preferred perspective in health economic evaluations [29]. This means that all costs are included: the direct medical costs, the indirect medical costs and the indirect costs associated with productivity loss of patients. Adopting a societal perspective also means that all rele- vant effects and all costs beyond the time frame of the trial should be measured. In this case, the differences between group training and waiting list can only be measured empirically till 13 weeks, but relevant effects and costs might occur beyond that artificial time horizon. For this reason, we will estimate effects and costs till 2 years, using a Markov model [30]. By making the Markov model prob- abilistic, we will be able to implement multiple sensitivity tests simultaneously and test for specific model assump- tions. A critical assumption will be the extrapolation of the effect beyond 13 weeks. This assumption will be explored by calculating the minimal duration that the group training should be effective, in order to achieve a satisfactory level of cost-effectiveness. Ethical considerations The Medical Ethical Committee of Erasmus Medical Center, Rotterdam, The Netherlands, has approved this study, registered under MEC-2004-191. Discussion The primary aim of our study is to investigate (cost-)effec- tiveness of an easily accessible protocollized group train- ing for patients with UPS in primary care in the daily practice of our mental healthcare institution, Riagg Rijn- mond, The Netherlands. Investigating the (cost-)effectiveness of our training in the daily practice of a mental healthcare institution has its benefits and its limitations. The huge advantage of inves- tigating the (cost-)effectiveness in the daily practice is that if (cost-)effectiveness is shown, the group training can be started without the delay of practical implementation issues. The limitations for this study are less possibilities for exclusion of patients and for the control condition. Page 7 of 9 (page number not for citation purposes)