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Chapter 9
Why and How
Figure 9-2
COMMENT
Dimension
In the absence of structured why details, a simple COMMENT dimension will still
allow BI users to search events using causal keywords and display comments on
reports when they find exceptional transactions. COMMENT dimensions can be
improved in future iterations by adding additional attributes and using “text-
mining” ETL routines to tag comments.
External Why Dimensions
WEATHER is an
external why
dimension. It could
be added to any
fact table with a
time and location
granularity that
matches the
weather feed
Figure 9-3 shows an example WEATHER dimension. This is an external why
dimension. It does not attempt to document every distinct temperature and
weather condition—it contains general weather descriptions that are useful for
reporting and analyzing non-weather events, not weather facts. Because weather is
dependent on time and location, any event with a where detail can potentially
include weather as a dimension; for example, events that might be usefully ana-
lyzed by weather include product sales, travel reservations, seminar attendance,
insurance claims, and television viewing. Adding a weather dimension requires an
external feed that matches the event’s time and location granularity.
Figure 9-3
WEATHER
Dimension
Causal dimensions—such as weather—that do not alter the granularity of fact
tables can be added later when a reliable external data source has been found.