The first step in choosing an aggregation function once the data structure is known is
usually to decide which class of either averaging, conjunctive, disjunctive or mixed
is desired. As discussed in Section 22.3.1.1, sometimes it will be more important
to have a function which sorts items into order of preference than one which gives
easily interpreted outputs. We consider four functions whose semantics can be used
to decide which class of function is required: