So, is this a new item and the old one
has ended, or is the size column simply changed?
As can be seen from these two simple examples, there??™s not necessarily an easy
answer. These issues represent what are called slowly changing dimensions. They
change slowly because people don??™t move from one manager to another with each
transaction. Item sizes don??™t change with each transaction (if they do, there are other
strategies, such as setting ranges of values and dropping each record into one of
those buckets).
There are different strategies for dealing with slowly changing dimensions. By
far the easiest way is what is called a Type I slowly changing dimension. With Type
I, history is simply overwritten. In the case of the salesperson, they??™d be tied to their
new manager and it would look like they had always worked for this manager; all
their history would now roll up to this new manager. This is great if the salesperson is
a stellar performer and the manager is the one getting this person, but it??™s horrible for
the manager losing this salesperson and having them replaced by a subpar performer.
38 B u s i n e s s I n t e l l i g e n c e w i t h M i c r o s o f t O f f i c e P e r f o r m a n c e P o i n t S e r v e r 2 0 0 7
In the case of a product, simply changing the size column in the database from
16 to 14 ounces makes it look like the size has always been 14 ounces. If sales
decline because of the change, it??™s entirely up to an analyst looking at the data to
remember when the change occurred and identify the change in size as an issue.
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