how to get median to work?

how to get median to work?

i can't seem to get the median custom formula to do anything. when i add it as an aggregate function, i get an error saying: 

" Sorry, cannot create a Aggregate Formula. You need to have an Aggregate function used in the formula.  For other types of formula, use "Custom Formula" option."

which is odd because median certainly feels like a function that would only apply in aggregate.

when i create it as a custom formula field, it never gets any value.