While browsing the forum for useful tips I spotted this formula posted
by Bob Phillips On Sept 21. It counts the number of unique records in a
list (A2:A20 in the example):
=SUMPRODUCT((A2:A20<"")/COUNTIF(A2:A20,A2:A20&""))

I found this formula very interesting and I just had to try it out. It
works, but I can't for the life of me understand how it works.
I've read the SUMPRODUCT Help file but that hasn't helped me.
COUNTIF Help file didn't help either. I can't see how A2:A20&"" works
as the Criterion for the COUNTIF part of the formula.
Can anybody help me understand Bob's incredible formula?