BMC Public Health 2007, 7:74 http://www.biomedcentral.com/1471-2458/7/74 (about the same as can be expected by giving a cholesterol lowering agent, i.e. a statin as medication), a standard deviation (sd) of 10%, an alpha of 0.05 (two-sided), and a power of 90% leads to 68 participants in each group. A multilevel analysis was performed to calculate the correla- tion of the outcome measures within practices. A correla- tion (r) of 20% was found and for that reason the number of participants has to be multiplied with a factor 0.8 (1 – correlation, to adjust for the block randomisation within general practices), leading to 68 patients (n) in each group. The formula used was [47,48]: n1 = n2 = 2(Z1-α/2 + Z1-β)2 sd2 (1-r)/d2 Z1-α/2 = Z (1 - 0.05/2) = Z 0.975 = 1.96 Z1-β = Z 0.90 = 1.282 sd = 0.10, r = 0.20, d = 0.05 n1 = n2 = 2 * (1.96 + 1.282)2 * (sd*sd)* (1-r)/(d*d) = 2 * 10.51 * (0.1 * 0.1) * (1 - 0.2)/(0.05 * 0.05) = 68 patients. Taking into account a possible drop-out rate of 30% the number of participants to be randomised must be 194 (97 in each group). Analyses Comparability between the two groups will be assessed at baseline. On the basis of an intention-to-treat analysis, differences in changes between the intervention and the control group are measured with 95% confidence inter- vals at 12 months for both primary and secondary out- comes. For dichotomous outcome variables a multilevel logistic regression analysis will be used. To calculate dif- ferences between continuous variables a multilevel analy- sis will be used, to adjust for the clustering of observations of patients receiving care from the same GP, and for repeated measurements within one patient. We will also perform a per-protocol analysis with patients that have attended at least three CBT sessions. We will adjust data for possible confounders: age, gender, weight at start of study, diabetes duration, level of educa- tion, depression, smoking, number of attended sessions, general practitioner, and type of health care provider (dia- betes nurse or dietician). Discussion This article provides a detailed description of the study design of an RCT on the added value of cognitive behav- ioural therapy focused on changing lifestyle in patients with type 2 diabetes and is published with two purposes. Firstly, to provide the opportunity for other researchers, health care providers and policy makers to critically review the methodological quality, the background theory and the practical issues of the RCT. Secondly, the publica- tion of this study design prevents publication bias. Studies with non-significant results are less likely to get published than studies with significant results. For that reason, the result of meta-analyses will be biased. The publication of our study design before the results become available will therefore prevent publication bias. The study becomes identifiable in search strategies for a systematic review and/or meta-analysis [49]. In addition, the authors bear more responsibility to publish the results, irrespective of the results appear to be positive or negative. We expect that the addition of a cognitive behavioural therapy to managed diabetes care will be effective in changing lifestyle in patients with type 2 diabetes. As a result of that, we expect that a changed lifestyle will cause a risk reduction in cardiovascular disease. The implemen- tation of such a study into an existing managed diabetes care system is unique. This system provides experienced medical assistants, diabetes nurses and dieticians and a high-quality infrastructure for patients with diabetes. One could argue, that, because the setting is well organised, the results of this trial cannot be easily generalised to all patients with diabetes. However, when the results appear to be negative, it is very unlikely that such an intervention will have success in a general setting. There are some limitations in the study design. We expect that patients who agree to participate are more motivated to change than patients who refuse to participate. How- ever, because the design of the study is a randomised con- trolled trial, this will be the case in both groups. Having a population of motivated patients might decrease the drop-out percentage. Patients in the control group may improve their lifestyle behaviour just by the knowledge that their outcomes are being used for research. However, this may also occur in the intervention group. Patients in the intervention group may benefit from the attention they get. However, because it is possible that patients in the control group receive follow-up visits as well, we can identify sub-groups based on number of sessions and compare the outcomes of the intervention and the control sub-groups in order to make a distinction between the effect of attention and the intervention. Another limitation of this study is the lack of a validated questionnaire for an important primary outcome meas- ure, the ASE-questionnaire. This questionnaire is sepa- rated into three dimensions (diet, physical activity and smoking). Because the use of validated questionnaires to assess the amount of physical activity, the diet behaviour and the amount of smoking, we can compare these out- comes with the ASE-questionnaire. We expected that these correlate with each other. It might therefore be possible to Page 8 of 10 (page number not for citation purposes)