I found this old note from 2013 in my files. SUMPRODUCT is a very useful formula, but not one I have used very much. Putting this here as a reminder of what it can do -- even though my example is probably not very helpfully written.
COUNTIF is fine where you want to count the number of items in one
column that meet one criterion. But what if you have two columns and two sets
of criteria? SUMPRODUCT.
(It's possible COUNTIFS might work for this example too: http://spreadsheets.about.com/od/excelfunctions/qt/20080127.htm)
