welcome to the forum, bibbi. you have the brackets at the wrong places. try:
Formula:
=SUMPRODUCT($L$4:$L$13,($K$4:$K$13="ABC")*(MONTH($M$4:$M$13)=B3)*(YEAR($M$4:$M$13)=B4)*($M$4:$M$13<>""))
and since you are checking for month in column M, you dont have to check if column M is blank.
Formula:
=SUMPRODUCT($L$4:$L$13,($K$4:$K$13="ABC")*(MONTH($M$4:$M$13)=B3)*(YEAR($M$4:$M$13)=B4))
alternatively without MONTH & YEAR:
=SUMPRODUCT($L$4:$L$13,($K$4:$K$13="ABC")*(TEXT($M$4:$M$13,"yyymm")=TEXT(B5,"yyymm")))
or even faster with Excel 2007 & above:
=SUMIFS($L:$L,$K:$K,"ABC",$M:$M,">="&B5,$M:$M,"<="&EOMONTH(B5,0))
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