Calculate the amount of sales to new customers per week

[#1]

I'm trying to calculate the amount of sales to new customers per week. But at the moment I only get the total sales per week.

count(if(OrderType='Sale', ItemId))-count(if(OrderType='Refund', ItemId))

[#2]

Here's the full question again.

I'm trying to calculate the amount of sales to new customers per week. But at the moment I only get the total sales per week.

count(if(OrderType='Sale', ItemId))-count(if(OrderType='Refund', ItemId))

I have the CustomerId as a dimension and also CalendarWeekOfYear.

Preferably I would want to have something like CalendarWeekOfYear()-1 in the expression above.