The joint decomposition of the Pietra index

In this paper, a multi-decomposition of the Pietra index is presented. This innovative methodology allows to achieve a relevant task, since it combines simultaneously the two most celebrated kinds of decomposition: by sources and by subpopulations. The key result of the proposed procedure is the detail level of decomposition: it allows to split the value of the index, by assessing the contribution of each source in each subpopulation. It is worth noting that this final result is not reached by all the decomposition procedures, since most of them do not permit to identify the contributions at so detailed level. The proposed joint decomposition is a sort of generalization, since from it two decompositions by sources and by subpopulations of the Pietra index, already proposed in the literature, can be obtained. Beyond the methodological details, an application based on the Survey Household Income and Wealth 2018 – carried out by Bank of Italy – is provided in order to clarify the advantages of the procedure.