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

Frank Mattes, MD			e-mail:	f.mattes at ucl.ac.uk