Help with a straight table calculation

[#1]

Greetings :

I have a straight table that looks like this :

SalesRep(Dim)	Year(Dim)	Month(Dim)	SalesLevelA(Exp)	SalesLevelB(Exp)	PreviousMonthSalesLevelA(Exp)
Bob	2013	12	100.00	200.00	Need sales form 11/2013 remove to keep it simple
Bob	2014	1	400.00	2300.00	100.00
Bob	2014	2	323.00	3233.00	400.00
Mary	2013	12	701.00	605.00	Need sales form 11/2013 remove to keep it simple
Mary	2013	1	323.00	415.00	701.00

My Question is how do I get the data to populate the PreviousMonthSalesLevelA(Expression).

When I do it in a set expression like: Sum({$<Posting_PeriodID={'=$(CurrentPeriodID - 1)'}>} Amount)

I end up getting a total on every row that matches August which right now is the current period -1.

Any help would be greatly appreciated.

[Gysbert_Wassenaar]

You can't use set analysis (at least not easily) this way. The set is calculated at the chart level, not row level. See this document for more information: Calculating rolling n-period totals, averages or other aggregations

[#3]

That is interesting, but I am still confused on how to tell it I do not want to accumulate but just display the previous rows amount.