Anthelmintic performance is frequently judged on the basis of percentage efficacy and most veterinarians are probably familiar with products claiming “reductions in worm burdens of better than 99%”. However, while there may be many who are acquainted with such claims, there are also likely to be similar numbers who may not realise that, impressive as they are, such efficacy figures may not always reflect practical reality or correspond with the high degree of anthelmintic effectiveness that they appear to imply. The reason for this is not because of any differences in performance against laboratory and field strains of parasites, but simply because of the way in which their efficacies may have been calculated.