assuming that the entries for each subject are ordered with respect to
TIME and each subject has a measurement at TIME = 0, then you could use
the following:
Data <- read.table(textConnection("ID TIME DV
1 0 0.880146038
1 1 0.88669051
1 3 0.610784702
1 5 0.756046666
2 0 0.456263368
2 1 0.369991537
2 3 0.508798346
2 5 0.441037014
3 0 0.854905349
3 1 0.960457553
3 3 0.609434409
3 5 0.655006334"), header = TRUE)
closeAllConnections()
Data$DVn <- with(Data, ave(DV, ID, FUN = function (x) x/x[1]))
Data
If either of the above assumptions doesn't hold, you'll have to tweak it.
I hope it helps.
Best,
Dimitris
On 6/20/2012 2:08 AM, york8866 wrote:
I have a dataframe such like the following:
ID TIME DV
1 0 0.880146038
1 1 0.88669051
1 3 0.610784702
1 5 0.756046666
2 0 0.456263368
2 1 0.369991537
2 3 0.508798346
2 5 0.441037014
3 0 0.854905349
3 1 0.960457553
3 3 0.609434409
3 5 0.655006334
. . .
. . .
I would like to generate another column with the normalized values of DV.
for each ID, normalize to the value at TIME 0.
I was able to use 2 loops to do the normalization, however, as 2 loops took
a long time for the calculation, I don't know whether there's a better way
to do it.
I have the following code with only 1 loop, but it did not work. Can anyone
help with this? Thanks,
IDS <- unique(data.frame$ID)
for(WhichID in IDS)
{
subset <- data.frame[,"ID"] == WhichID
DVREAL <- data.frame[subset,"DV"]
DVNORM[WhichID] <- DVREAL/DVREAL[1]
}
--
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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--
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center
Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014
Web: http://www.erasmusmc.nl/biostatistiek/
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
