I think this ought to be easy, but have failed to come up with any
ideas.
I have adjacent columns entitled "OK" and "Failure reason".
OK entries can have data validation to by Yes or No
Failure reason should be empty for OK = Yes
Failure reason should not be empty for OK = No

I could construct a VBA function to enforce such constraints,
but am looking for a solution without VBA.

I would prefer a solution which does not use auxiliary columns.

Happy 2015 to all readers of microsoft.public.excel.worksheet.functions.
--
Walter Briscoe