Anti-parasitics are an important class of therapeutics in the animal health industry for both companion animals (cats, dogs, horses etc.) and livestock (cattle, swine, sheep, poultry etc.). Parasites of interest may be external (e.g. fleas, ticks, mites, parasitic flies etc.) or internal (intestinal worms, heart worm, etc.). Efficacy is typically evaluated by the percent reduction in parasite count relative to a contemporary negative control or a pre-treatment count, in the absence of a negative control. The choice of statistic to use to estimate parasitic burden in the evaluation of efficacy (typically arithmetic mean or geometric mean) may depend on the parasite type (internal or external), the study type (laboratory or field) and also the objectives of the study. Considerations for study design, sample size and statistical analyses used (typically general mixed linear model, generalized mixed linear models or non-parametric) will be discussed.