Optimizing Animal Randomization for In Vivo Experiments v1

This protocol describes a standardized procedure to distribute laboratory animals (mice, rats) into experimental groups using the online tool Randmice (https://randmice.com). The algorithm minimizes inter-group heterogeneity by iteratively testing combinations of animal assignments and selecting the distribution with the lowest covariate imbalance. This approach reduces operator bias, improves reproducibility, and supports compliance with the 3Rs principle (Replacement, Reduction, Refinement) as required by European Directive 2010/63/EU. The tool is optimized for small-scale experiments (N < 12 animals per group), where random assignment leads to high inter-group heterogeneity (Bertsimas et al., 2015).