Thanks Tom, I've got a new wrinkle:

Student Status Marks
Joe ENROLLED 20
George NOT 10
Dick ENROLLED DNS
Alan ENROLLED 10

=AVERAGEA(IF(Status="ENROLLED",Marks))

as an array formula works fine - it handles the "DNS" as a zero.

=STDEVA(IF(Status="ENROLLED",Marks))

fails. It does not count the DNS as a zero. It acts like STDEV.
If I replace the cell containing DNS with zero, then it calculates
differently.

Any wisdom appreciated.

Thanks