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calculating means per group

4 messages · RFTW, ONKELINX, Thierry, Kjell Konis +1 more

RFTW
# 
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
ONKELINX, Thierry
# 
Dear Luc,

You should send this kind of questions to the general mailing list
(r-help at r-project.org) instead of the developer list.

To answer your question: have a look at ?by and ?aggregate

HTH,

Thierry 


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium 
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be 
www.inbo.be 

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-devel-bounces at r-project.org
[mailto:r-devel-bounces at r-project.org] Namens RFTW
Verzonden: donderdag 4 september 2008 9:07
Aan: r-devel at r-project.org
Onderwerp: [Rd] calculating means per group


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
Kjell Konis
# 
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:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Brian Ripley
    

    

      
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On Thu, 4 Sep 2008, ONKELINX, Thierry wrote:

        

            

      
      
      
    

        
And ?ave .