Measure Forecast Accuracy Several Months

Hello,

I'm trying to develop a PowerBI Dashboard to measure the forecast accuracy:

My datastructure is like this in one Table (Several Countries, several Articles and Several Months)

If I want to measure the different Forecast KPI it works perfectly for single months:

BIAS (The Difference between Sales and Plan). The measure is: SUM(Table[Month_Forecast]) - Sum(Table[Month_Sales])

MAD(The Sum of the absolute difference of each article line). The measure is = SUMX(Table;ABS(Table[Month_Forecast] - Table[Month_Sales]))

If I want to measure the same KPI for several months (in this case the months 10.2019 - 12.2019) I don't know how to measure the MAD:

BIAS ==> The same DAX expression like for single months works perfectly

MAD==> If I am using the same expressions like for single months it shows me a MAD of 900 pcs for country DE (The measue calculates the sum of the ABS difference per each article and month!)

What the measure should do is to calculate the absolute difference per article for the whole selected time range (10.2019 - 12.2019): An Example:

• Article 1 has a forecast of 1000 pcs for the selected time range in country DE and sales of 900 pcs = MAD of 100 pcs for this article 
• Article 2 has a forecast of 900 pcs for the selected time range in country DE and sales of 500 pcs = MAD of 400 for this article

==> So the MAD for the country DE in the time range 10.2019 - 12.2019 should be 500 pcs, but I don't know how to generate the Measure.

I hope someone can help.

Many Thanks in advance,

Mario