calculating means per group
Hi Luc,
First of all, questions like this should really be asked on the R-help
mailing list.
The tapply function does what you want:
> year
[1] 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2
Levels: 1 2
> area
[1] a a a a a b b b b a a a a b b b b b
Levels: a b
> value
[1] 20 25 28 31 23 25 28 23 19 25 23 24 26 27 28 20 25 28
Note that both year and area are factors.
Get the mean for each area:
> tapply(value, area, mean)
a b
25.00000 24.77778
If you make the second argument a list then you can subset on both
factor columns:
> tapply(value, list(year, area), mean)
a b
1 25.4 23.75
2 24.5 25.60
Kjell
On 4 sept. 08, at 09:06, RFTW wrote:
Hi all I have a very basic question, yet i have not found how to do it. Suppose my dataset looks like this: Year Area value 1 a 20 1 a 25 1 a 28 1 a 31 1 a 23 1 b 25 1 b 28 1 b 23 1 b 19 2 a 25 2 a 23 2 a 24 2 a 26 2 b 27 2 b 28 2 b 20 2 b 25 2 b 28 Now, i want to calculate a MEAN per year per area. How do i do that? With mean(value) i calculate the mean of all values of course. I just need to know how to group year and area correctly. I assume that i can use this grouping in other calculations too, right? Cheers, Luc -- View this message in context: http://www.nabble.com/calculating-means-per-group-tp19271479p19271479.html Sent from the R devel mailing list archive at Nabble.com.
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