One of the biggest problems in applications of animal welfare science is our ability to make
comparisons between different individuals, particularly different species. Although welfare science provides methods for measuring the welfare of individual animals, there’s no established method for comparing measures between individuals. This problem occurs because of the underdetermination of the conclusions given the data, arising from two sources of variation that we cannot distinguish – variation in the underlying target variable (welfare experience) and in the relationship of measured indicators to the target. In this paper I describe the similarity assumptions that underlie our current applications of interspecies comparisons and examine in which cases they are justified, as well as describing alternative methods we may use when they are not. In the end, all our available options for making interspecies comparisons are imperfect, and we need to make explicit context-specific decisions about which will be best for the task at hand while acknowledging their potential limitations. Future developments in our understanding of the biology of sentience will help strengthen our methods of making welfare comparisons.