Sample size
The a priori sample size calculation assumed 90 % follow-up retention and used a simplifying t-test for power calculations. We found that a sample size of 90 per study arm, chosen for logistical feasibility, would yield 80 % power to detect a difference in the number of drinking days in the prior 21 between two study arms at follow up of ≥ 2.4 days, i.e. nearly 1 fewer day per week, as statistically significant (α = 0.05) assuming that the mean number of drinking days in the prior 21 in the control group at six months would be 7, based on prior data.