Rich wrote...
....
=SUMPRODUCT(
--(Projects_State=F15),
--(Projects_Dev_Type=F17),
--(Projects_Carrying_Value=F19),
--(Projects_Carrying_Value<=F21))
)
....
The formula works fine. *However, I want cells F15 and F17 to be "optional"
whereby if a user *does not* enter a value, it is treated as if all values
are required.

One way

=SUMPRODUCT(
--((Projects_State=F15)+ISBLANK(F15)0),
--((Projects_Dev_Type=F17)+ISBLANK(F17)0),
--(Projects_Carrying_Value=F19),
--(Projects_Carrying_Value<=F21))
)