[R] Normalizing grouped data in a data frame

[Sandy Small]

Hi
I am a newbie to R but have tried a number of ways in R to do this and 
can't find a good solution. (I could do it out of R in perl or awk but 
would like to know how to do this in R).

I have a large data frame 49 variables and 7000 observations however for 
simplicity I can express it in the following data frame

Base, Image, LVEF, ES_Time
A, 1,  4.32, 0.89
A, 2, 4.98, 0.67
A, 3, 3.7, 0.5
A, 3. 4.1, 0.8
B, 1, 7.4, 0.7
B, 3, 7.2, 0.8
B, 4, 7.8, 0.6
C, 1, 5.6, 1.1
C, 4, 5.2, 1.3
C, 5, 5.9, 1.2
C, 6, 6.1, 1.2
C, 7. 3.2, 1.1

For each value of LVEF and ES_Time I would like to normalise the value 
to the maximum for that factor grouped by Base or Image number, adding 
an extra column to the data frame with the normalised value in it.

So for the Base = B group in the data frame (the data frame should have 
the same length I'm just showing the B part) I would get a modified data 
frame as follows.

Base, Image, LVEF, ES_Time, Norm_LVEF, Norm_ES_Time
...
B,1,7.4, 0.7, 7.4/7.8, 0.7/0.8
B, 3, 7.2, 0.8, 7.2/7.8, 0.8/0.8
B, 4, 7.8, 0.6, 7.8/7.8, 0.6/0.8
...

Where the results of the division would replace the division shown here.
I hope this makes sense.
If anyone can help I would be very grateful.

Sandy Small
NHS Glasgow, UK


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