Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 218 / 3 / e / Solution

The fixed-effect correlation pattern changes only moderately after adding the player random intercept: the competition-B/competition-C correlation remains $0.488$, rival S remains essentially orthogonal to those competition contrasts, and the largest changes involve correlations with the intercept. Thus the principal coefficient dependence comes from the fixed-effect coding and design, while player heterogeneity mainly changes the intercept-related uncertainty. The random effect is scientifically useful without materially changing which fixed contrasts are confounded.