On Sat, 3 Mar 2012 19:32:46 -0800, "joeu2004" wrote:

No need to abandon SUMPRODUCT. Simply write:

=SUMPRODUCT(--(MOD(ROW($A$1:$A$1000)-FirstRow,3)=0),$A$1:$A$1000)

The double negative converts TRUE and FALSE to 1 and 0, which SUMPRODUCT
requires, as the multiplication did before.

Of the behavior of the double unary I was well aware. What I did not realize is that:

=SUMPRODUCT(0*"x") -- #VALUE!
=SUMPRODUCT({0}*{"x"}) -- #VALUE!
=SUMPRODUCT(0,"x") -- #VALUE!

but

=SUMPRODUCT({0},{"x"}) -- 0