How many subjects are enough in a veterinary trial?—Literature review and insights from industrial statisticians

To approve veterinary medicines, regulatory agencies require substantial evidence of safety and effectiveness from well-designed trials. A critical consideration in planning a veterinary trial is to determine the requisite number of subjects needed to achieve the trial objective(s). Sample sizes for preclinical studies usually follow specific guidelines, whereas clinical studies should follow more rigorous requirements to determine the optimal sample size for detecting significant and meaningful treatment effects. This paper presents a descriptive literature review aiming to explore current reported practices in sample size calculation for published veterinary clinical trials. The review included articles published in 12 top-ranking veterinary health journals from January 1, 2013, to June 4, 2023, sourced from PubMed and Medline regarding animal trials. Initially, 294 articles were identified, of which 98 (33 %) focused on veterinary clinical studies. Among these 98 papers, only 24 (24 %) provided detailed methods on sample size calculations, with individual animals as experimental units across at least two arms. The predominant animal species studied in these trials were canines (39 %), cattle (32 %), and swine (21 %), with most studies aiming for a statistical power of at least 80 %. Unlike human clinical trials, the paper found that statistically rigorous sample size calculations were less commonly reported in animal clinical trials. Our paper provides recommendations for veterinary clinical trial practitioners and offers insights into how sample size determination can be properly conducted and reported. Furthermore, this article extends to discuss practical issues in sample size determination for preclinical studies.