We discuss various decomposition schemes for analysing the dielectric constants of Zr silicates in terms of local properties. Such schemes serve the purpose of predicting the dielectric constants of amorphous alloys, when their system size precludes the possibility of performing accurate first-principles calculations. In particular, we compare two decomposition schemes which have found application in the recent literature. The first scheme is based on a decomposition into basic structural units characterized by effective parameters. While this scheme was originally developed for cation-centred structural units, we here also consider its application to anion-centred structural units. In the second scheme, the difference between the static and optical dielectric constants is formally decomposed into atomic contributions. We analyse the results of these two schemes when applied to a set of (ZrO2)(x) (SiO2)(1-x) model structures, for which the dielectric properties are computed from first principles. The most promising results are recorded for the first scheme when applied to cation-centred structural units.