The number of case managers actually recruited was 79 which was large enough to detect a standardised effect of 0.65 on the Knowledge about Dual Diagnosis (KADD) questionnaire.
Assuming an initial mean symptom score of 250 (SD 80) in all groups, 35 patients were required in each group to detect a difference in score of 53 points (a moderate standardised effects size of 0.66) between the treatment groups (mebeverine, methylcellulose and placebo) or the three website groups (none, minimal support, telephone) for 80% power and 95% confidence.
This bias translates to a standardised effect size of 0.4 (0.4 of the baseline standard deviation).
A sample size of 64 participants per group (DA, control), per cohort (DCIS, Prevention) yields 80% power to detect a moderate (standardised) effect size of 0.5 in the primary outcome of decisional conflict or any of the secondary outcomes, assuming a two-sided alpha of 0.05 (Cohen, 1992).
The original sample size calculation of 450 participants with valid outcome data at 3 months post-randomisation, was based on detecting a standardised effect size of 0.33 or a mean difference of one-third of a standard deviation in the outcome measures between the booster and control groups.
The original sample size calculation of 450 participants with valid outcome data at 3 months post-randomisation, was based on detecting a standardised effect size of 0.33 or a mean difference of one-third of a standard deviation in the outcome measures between the intervention and control groups.
We based our sample size calculation on 90% power, a two-sided α = 0.05 t-test, dropout of 30%, a difference between groups of 14.5, and a standard deviation of 29 [ 39] (a standardised effect size of 0.5).
These statistics indicate a smaller standardised effect size of interest (Cohen's d) of 0.5 for the WOMAC measure than the d of 0.6 for pain.
A previous small study using a similar intervention found a mean benefit of 0.7 in the AIMS2 with a standard deviation of 1.81 and a standardised effect size of 0.39 [ 36].
