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AbstractIn this paper we survey on aggregation operators and in particular hierarchies of them. As a case study, we will analyze the problems of the aggregation of truth values of fuzzy predicates and the aggregation of individual opinions into a single group opinion, based upon hierarchical intensity aggregation rules. We will see that hierarchical amalgamations are supported from an ethical and rational point of view. Two different hierarchical procedures will be recalled: cover-based hierarchical aggregations and ordered hierarchical aggregations. Finally we will see that when we deal with ordered hierarchical aggregations of OWA operators, some interesting computational problems appear quite naturally. Such problem admit polynomial time solutions.