Sumif Logical Formula PowerPivot

[jenneki]

Please help,

Say I have the following rows of data:

[TABLE="width: 500"]
<tbody>[TR]
[TD][/TD]
[TD]A[/TD]
[TD]B[/TD]
[TD]C[/TD]
[TD]D[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]Year
[/TD]
[TD]Week[/TD]
[TD]Sales[/TD]
[TD]Prior Week[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]2011[/TD]
[TD]1[/TD]
[TD]10[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]2011[/TD]
[TD]2[/TD]
[TD]15[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]2011[/TD]
[TD]3[/TD]
[TD]20[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]5[/TD]
[TD]2012[/TD]
[TD]1[/TD]
[TD]5[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]6[/TD]
[TD]2012[/TD]
[TD]2[/TD]
[TD]25[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]



In my "Prior Week" Column I'd like to create a simple formula which in excel would have read SUMIF(A2:A6,B3-1,C2:C6). For the life of me, I can't figure out how to do it in Powerpivot. I've tried all iterations of Calculate, Filter and All I've found on the internet, but nothing works.

Any assistance would be greatly appreciated.

Thanks.