Sample size
Recruitment will be split equally across centers. In a conservative fashion we power the study to detect moderate effect sizes. In the pilot study the effect sizes were large: worry (PSWQ mean difference = 10.00 SD = 9.50) = 1.05; persecutory delusion (PSYRATS Mean difference = 2.91 SD = 2.15) = 1.35. A simple two-tailed t-test with 60 people per group would provide 90% power to detect an effect size of 0.60 at a significance level of .05. It would have 80% power to detect an effect size of 0.52. In practice, further power will be gained by use of multiple regression. Drop-out from the assessments in the pilot was low - 13%. The intervention is brief and the time in the trial will be relatively brief (6 months). Therefore conservatively allowing for 20% drop-out, 150 people will need to be recruited to enable full data to be obtained from 120 participants (60 in each condition).