Measures are the numeric values that our users want to aggregate, slice, dice and otherwise analyze, and as a result, it's important to make sure they behave the way we want them to. One of the fundamental reasons for using Analysis Services is that, unlike a relational database, it allows us to build into our cube design business rules about measures: how they should be formatted, how they should aggregate up, how they interact with specific dimensions, and so on. It's therefore no surprise that we'll spend a lot of our cube development time thinking about measures.