Calculating aggregate tables in BISM Tabular

One of the most interesting functions in Denali's DAX implementation is the Summarize() function. To see how this can be of great use, imagine the following table called Payments:

Suppose we wanted to know what the employee monthly payment average is. To calculate this, we decide we might want to start by calculating the monthly payment average as a first step. In order to do this, we must first add the values for each month and then calculate the average. So we try the following DAX query in SSMS: