Full model equations are provided in the Text S1. APOE genotype was treated as a categorical variable with four levels: ε2/ε3, ε3/ε3, ε3/ε4, and ε4/ε4. The ε3/ε3 genotype served as the reference group for comparison. Age was split into three groups (< 60, 60−69, and 70+) based on sliding‐window results. The youngest age group (< 60 years) served as the reference category for the remaining groups (60−69 and 70+). Categorizing age allows us to account for its non‐linear association with hippocampal volume while categorizing APOE by genotype avoids assuming an additive linear effect of ε2 or ε4 allele dosage. This approach also facilitates the interpretation of results. See Table S7 in supporting information for details on the data type and variable encoding of each predictor. The goodness of fit between the two models was assessed using a likelihood ratio test (LRT).
