[R] tapply problem

[Robin Hankin]

Hello everybody.

I have a question that has stumped me and the usual "apply" tricks
don't seem to work.  I run a course where each student's performance
is marked by one or more assessors. 

I have a data frame containing students' names, assessors' names and
their marks, arranged as follows:


        ID       student   assessor  Q1A  Q1B  Q1C  Q2A  Q2B  Q3
1  2152833    Fred.Smith      Robin 15.0 17.0 13.0 14.0 13.0  8.0
2  2152833    Fred.Smith      Steph 14.0 13.0 11.0 16.0 13.0  8.0
3  2152833    Fred.Smith      Julie 15.0 17.0 13.0 17.0 13.0  8.0
4  9821091    Joe.Bloggs       John 13.0 12.0 12.0 15.0 12.0  8.0
5  9821091    Joe.Bloggs      Julie   NA   NA   NA 18.0 14.0  8.0
6  9821091    Joe.Bloggs      Robin 12.0 12.5   NA 16.0 13.5  8.0
7  9821091    Joe.Bloggs      Steph   NA   NA 12.0 17.0 12.5  NA
8  9791734     Bob.Jones      Julie 13.0 12.0  8.0 16.0 13.0  8.0
9  9791734     Bob.Jones      Robin 11.0 12.0  7.0 14.0 11.0  8.0
...


note that some students are marked by more assessors than others.


What I want is each student's mean mark for each question, averaged
over the appropriate assessors.  Is there a neat vectorized method I
could use?

The best I can come up with is "tapply(x$Q1A,x$name,mean,na.rm=T)" but
this works question-by-question and I would like a reduced data frame
with the same column headings.


this is driving me crazy; thanks in advance!



-- 

Robin Hankin, Lecturer,
School of Environmental and Marine Science
Private Bag 92019 Auckland
New Zealand

r.hankin at auckland.ac.nz
tel 0064-9-373-7599 x6820; FAX 0064-9-373-7042



--f9295XO03655.1002013533/r.hankin.sems.auckland.ac.nz--



-- 

Robin Hankin, Lecturer,
School of Environmental and Marine Science
Private Bag 92019 Auckland
New Zealand

r.hankin at auckland.ac.nz
tel 0064-9-373-7599 x6820; FAX 0064-9-373-7042


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