Sample size The sample size was calculated on showing a difference in the post-treatment PANSS positive symptoms score between groups. A previous study (reference in here?) found a standard deviation for the post-treatment scores of 5.8 in the control group, and 1.8 in the CBT group. In the analysis, the pre-treatment scores will be adjusted for, and the previous study found a correlation of 0.2 between the pre and post treatment scores. A difference between groups of 15% of the pre-treatment values would be regarded as being of clinical importance. This equates to a difference of 3 units on the PANNS positive symptoms score scale. Using a 5% significance level and 90% power, it is calculated that 41 subjects per group are required, 82 in total. It is expected that up to 30% of patients may drop-out of the study. To allow for this, a total of 120 patients will be recruited into the study.