Quote Originally Posted by ChemistB View Post
Ahh, it's because you don't have February, you have Feb. This modification should work

=IFERROR(SUMPRODUCT(--(TEXT($A$2:$A$19,"mmm")=D2),$B$2:$B$19)/SUMPRODUCT(--(TEXT($A$2:$A$19,"mmm")=D2)),"")

changed from "mmmm" which expects the entire month to be written out to "mmm" which expect the 3 character abbreviation. Also I added the IFERROR part to keep out the errors when you don't have a month listed at all.
That worked, however, this was a sample and the rather large document I am working on has some blank cells (not yet entered). The formula is calculating these blank cells as "zeros" and lowering the actual average. I am in over my head!