EXPERT RESPONSE
I'm always interested primarily in the data underlying any online analytical processing (OLAP) project. When you say the source is Excel, I think about how difficult it is to get users to depend on a locally owned and inconsistently changed spreadsheet. You will want to make sure you have repeatability in the underlying data and can express that to the anticipated users. As for aggregations, and derivations for that matter, my rule of thumb has to do with usage and performance. I think about if the users can easily get to what they ultimately need. If it's a straight rollup (summarization), the OLAP tool should handle that pretty easily. However, if it's going to be a frequently used summary, you still may want to do it as part of the OLAP build cycle. Occasionally, I'm convinced only these summarizations are needed, but usually there is value in having the detail available through OLAP as well. Certainly, if you are deriving new fields for the OLAP, bring over the underlying components of the calculation as well.
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