Though it cannot be summed up, it can still be aggregated by
using operators such as average, maximum, and minimum.
Since additivity is the most frequent case, in order to simplify the graphic notation in
the DFM, only the exceptions are represented explicitly. In particular, a measure is
connected to the dimensions along which it is nonadditive by a dashed line labeled
with the other aggregation operators (if any) which can be used instead. If a measure
is aggregated through the same operator along all dimensions, that operator can be
simply reported on its side (see for instance unit price in Figure 4).
Approaches. to. Conceptual. Design
In this section we discuss how conceptual design can be framed within a methodology
for DW design. The approaches to DW design are usually classified in two
categories (Winter & Strauch, 2003):
??? Data-driven (or supply-driven) approaches that design the DW starting from
a detailed analysis of the data sources; user requirements impact on design by
allowing the designer to select which chunks of data are relevant for decision
making and by determining their structure according to the multidimensional
model (Golfarelli et al., 1998; H??semann et al., 2000).
??? Requirement-driven (or demand-driven) approaches start from determining
the information requirements of end users, and how to map these requirements
onto the available data sources is investigated only a posteriori (Prakash &
Gosain, 2003; Schiefer, List & Bruckner, 2002).
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