Period over Period Change, Filtered by Current Attribute

[amstreit92]

I am trying to create a formula to find the monthly change in aggregated sales data.  But I would like to exclude any data from the previous month that does not compare with the current products in production. 

For example, Product A was dropped from production in the current month (1/31/2017).  I'd like the formula to compare the monthly change in product B and C, and exclude Product A. The formula must recognize that Product A data was not included in the current period and filter it from the total monthly change.

Product	12/31/2016	1/31/2017	Monthly Change
A	100		-100
B	200	250	50
C	300	350	50

[v-caliao-msft]

@amstreit92,

Could you please provide us some sample data, so that we can make further.

Generally, we can get previous month amount by using PREVIOUSMONTH function. It returns a table that contains a column of all dates from the previous month, based on the first date in the dates column, in the current context. Sample DAX expression.
=CALCULATE(SUM(InternetSales_USD[SalesAmount_USD]), PREVIOUSMONTH('DateTime'[DateKey])) 

Regards,

Charlie Liao