I was testing some calculations in my tabular model when I noticed that my Lost Customers calculation wasn’t working as expected. This was rather interesting to me since the calculation I was using was from DAXPatterns.com. After some experimentation, I determined that the difference between my data situation, and the situation that fit the DAX Pattern was that my customer dimension was a Type 2 Slowly Changing Dimension. That meant I couldn’t use the customer key to identify a unique customer, since each customer could have multiple customer keys (surrogate keys) with the same customer number (business key). Specifically, if a customer made a purchase in December 2015, then changed their name, then made a purchase in January and February, my calculation was counting them as lost because it was not recognizing that the customer with the new name and same customer number was actually the same customer.
She ends up with two solutions, each with different trade-offs. Knowing as little DAX as I do, looking at two different ways of solving this problem is great because it gives you more insight into language semantics.