Grouping by an Aggregation in DAX

Business users really like the option of comparing category members against the average for all. Let's review how to accomplish this in DAX.
Suppose you what to find out which months had an above average sales performance on the data above (tables come from AdventureWorksDW). We must first create a new table in the model, to hold the categories we want. In PowerPivot, these values can imported as a linked table: