I WOULD like to congratulate the authors of the paper on whole-flock metaphylactic treatment of contagious ovine digital dermatitis (CODD) with tilmicosin (Angell and others 2016) for performing what must have been a vast and complex undertaking. However, there are a couple of issues that the study brings up which are skated over in the published paper. First, and highly commendably, the authors undertook a prestudy power analysis. However, this was not a power analysis for their primary outcome – elimination; it was for a secondary outcome – reduction in prevalence (an outcome for which results weren't detailed in the abstract). While under some circumstances elimination per se may not suit a power analysis (although in this case comparing proportions of farms with disease at study end …