We want to find how often customers are behind in their payments or calculate their average debt when they are in debt.  The question might be

1. How many customer-months of debt am I experiencing (counting 1 for each month that each customer is in debt).
2. What is the average monthly debt of customers (aggregating only months where a customer is in debt).

And similar questions, such as profit in profitable-months.

The aggregations (i.e. COUNT or AVERAGE) should be partitioned by

1. customer
2. time interval (e.g. month)

and there will need to be some form of conditional clause to return a value only when the customer’s balance is above zero (debt).

Debt in indebted-months = SUMX(
	CROSSJOIN( 
		VALUES( 'Date'[Month] ), 
		VALUES( 'Customer'[CustomerId] ) 
	),
	VAR customer_balance = [Customer balance]
	RETURN
	IF (
		customer_balance > 0,
		customer_balance
	)
)

Performance

How can performance be improved? 

1. The most obvious optimisation is already in place:  [Customer balance] is assigned to a variable, which avoids calculating it twice for indebted customers.
2. The number of iterations will be the product of Customers and Months, so if these dimensions are small (a restricted date range is in context) then performance might be OK.
3. If there is no filter on date in context then it might help to filter the date table to include only months where there is at least one customer balance.

Anything else?

• Can we avoid using IF? Is there an expression using only mathematic functions that will return only positive values?  (A limitation here that I can see is that math expression will return zero where the balance is <= 0 which will be OK for some uses but not when calculating average.)
• Is the CROSSJOIN optimal?  And the use of VALUES to return tables? For Date, the VALUES clause returns a table containing the set of months, but for Customer the VALUES clause returns the table grain, so the table itself could have been used: do those extra columns have a cost where they are not referred to? Something to investigate.