Hi Sandy,
Excel doesn't currently have the equivalent of a STDEVIFS() function, so you would either have to use a helper column or an array formula to get the same effect.
For example, if you have these values in F4:G9
Code:
a 1
a 3
b 4
a 5
a 6
Then, this array formula (CTRL+SHIFT+ENTER) would give you the STDEV corresponding to "a"
Code:
=STDEV(IF(F4:F8="a",G4:G8))
This formula gives the same result as
Code:
=STDEV(G4,G5,G7,G8)
Does that help?