Heart weight (HW) is a critical parameter in cardiology and mouse
research, commonly normalized to body weight (BW) or tibia length (TL)
to account for size differences. Ratio-based normalization, however,
assumes strict proportionality between variables, an assumption that is
rarely tested and may bias group comparisons. We analysed HW, BW, and TL
measurements from over 25,000 C57BL/6N wildtype mice generated by the
International Mouse Phenotyping Consortium. Sex- and age-stratified
analyses were combined with simulation-based modelling to evaluate
empirical scaling relationships and the statistical behaviour of
ratio-based normalization. Across all age and sex groups, correlations
between HW, BW, and TL were negligible to weak, indicating substantial
deviations from proportionality. Simulations demonstrated that
ratio-based normalization can generate misleading results, including
spurious or reversed group differences, when proportionality assumptions
are violated. Ratios were consistent with linear and allometric models
only under strictly proportional conditions, characterized by regression
lines passing through the origin. Linear models with covariate
adjustment and allometric scaling provide more robust and biologically
meaningful frameworks for organ weight analysis. Ratio-based
normalization should be avoided unless key mathematical assumptions are
met.