Average farm-level prevalence of anthelmintic resistance in gastrointestinal nematodes in sheep and goats, by decade. The arithmetic mean was taken of all reports including more than one farm, from countries with a minimum of three reports in different years, excluding Italy (see text). BZ = benzimidazoles, n = 64; LEV = levamisole, n = 14; ML = macrocyclic lactones excluding moxidectin, n = 23; MOX = moxidectin, n = 10. Error bars are standard deviations.