An Alternative to a physical Distinct Count Measure

Many SQL Server OLAP developers learn the hard way that a DistinctCount measure can be expensive on performance - in query time, processing time and cube size. A Distinct Count measure might be appropriate for your cube, but you should be aware that every cell, and every aggregation will need to contain all the distinct values at that intersection. That's why a physical DistinctCount measure can be so expensive. It's also why there is a lot of literature on how to optimise the performance of a Distinct Count measure.

That is not to say that you shouldn't use DistinctCount, it can work well and be extremely useful. However, you should be aware that there is an alternative, which has its own pros and cons.