building mean/median over subgroups

Dear all,

I'm trying to solve the following problem, and hoping to get some advise 
here from the group

I have a dataframe in which the same sample was measured more than once 
on the same day. I would reorganize the dataframe to get a single value 
(mean /median) for one day

Patient    Day    Sample    Test
A    1    A    23
A    1    A    36
A    5    B    44
A    5    B    23
B    2    C    10
B    2    C    5


mean

Patient    Day    Sample    Test.mean
A    1    A    29.5
A    5    A    33.5
B    2    C    7.5


it would be great to get some help form the list.

Yours
Frank