Hi,
Look at ?aggregate.
df <- read.table(textConnection("Year Day Time herring.density
2007 47 10.36 2.2
2007 47 11.50 1.1
2007 47 14.24 1.4
2007 66 9.35 2.5"), header=TRUE)
You could do this:
aggregate(herring.density~Year+Day, data=df, FUN=mean)
And for the second question:
aggregate(herring.density~Year+Day, data=df[df$Time>=10 & df$Time<=16,],
FUN=mean)
Alternatively, you can use the function summaryBy in the doBy package.
You use it exactly the same way as aggregate. You'll only get different
column names.
HTH,
Ivan
Le 3/16/2011 11:17, OA Fatnes a écrit :
Hi,
I have a dataset with many observations some days while only one others. I
would like to calculate a mean value per day and then do regression analysis
on the means.
This is what I have:
Year Day Time herring.density
2007 47 10.36 2.2
2007 47 11.50 1.1
2007 47 14.24 1.4
2007 66 9.35 2.5
This is what I want:
Year Day herring.density
2007 47 1.57
2007 66 2.25
I would also might like to extract means between time 10-16 (h) so that
Year Day herring.density
2007 47 1.25
Any idea on how to do this?
I have tried
means07<-tapply(herring.density,Day,mean)
Then I get means for every day, but means07 won't fit when plotted against
days, because it is now shorter than Day. I also get a lot of NA values
where there are only 1 observation that day...
Help would be much appreciated!
Cheers,
Ole Andreas
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--
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
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D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
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**********
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