Finally, we have shown that approximate rattling frequencies, determined from the harmonic force constants, appear to be a useful predictor of the 𝜅 latt in the filled skutterudites. In particular, rattling frequencies in the range of the acoustic modes, obtained with heavy fillers that interact weakly with the CoSb3 framework, are predicted to lead to the largest suppression of the thermal transport. Identifying descriptors to predict the interaction strength between the fillers and the framework (e.g. charge, electronegativity, and polarizability) is therefore likely to be a useful step towards the rational selection of fillers for controlling the heat transport in cage compounds.