The τ CRTA can be further written as the product of a set of averaged modal (anharmonic) three-phonon interaction strengths, P λ , and a counting function N 2 (q q q, ω) that captures the number of allowed energy-and (crystal) momentum-conserving scattering pathways (termed the "scattering phase space"). A comparison between systems can be made by calculating a weighted-average interaction strength, P, and a function N2 that shows the size of the scattering phase space (average number of scattering pathways) as a function of phonon frequency only. Full details can be found in our previous work. The P were determined at 800 K as described in our previous work (Figs S25 -S28, ESI † ). Scaled P and N2 that that can be compared between systems are provided in Table and The calculated P of the Pnma Ge and Sn chalcogenides range from 1.10 -3.06 × 10 -8 eV 2 . The averaged interaction strengths are slightly higher in the Ge chalcogenides, and around 2× larger in the sulphides than the selenides. The calculated P of R3m GeSe is ∼30% larger than that of Pnma GeSe, but the averaged interaction strength in rhombohedral GeS is a much larger 2× that of Pnma GeS. Taking the P as a measure of the intrinsic phonon anharmonicity, this analysis shows that the sulphides are more anharmonic than the selenides, the Ge chalcogenides are more anharmonic than the Sn analogues, and the R3m phases are more anharmonic than the Pnma structures.