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How to resample one per group

2 messages · Johannes Radinger, ONKELINX, Thierry

Johannes Radinger
# 
Hello,

I have got a dataframe which looks like:

y <- c(1,5,6,2,5,10) # response
x <- c(2,12,8,1,16,17) # predictor
group <- factor(c(1,2,2,3,4,4)) # group
df <- data.frame(y,x,group)

Now I'd like to resample that dataset. 
I want to get dataset (row)
per group. So per total sample I get 4 rows
into a new data frame. How can I do that?
Is there any simple approach using an existing
package.

I looked at function strata() from package sampling.
I don't if that is the function for that or
if there is a simpler approach with sample().

What I unsuccessfully tried so far:

library(sampling)
strata(data=df,group,size=(rep(1,nlevels(group))))

Maybe you can help me to do this resampling...

Thank you,
Johannes
--
ONKELINX, Thierry
# 
Something like this?

library(plyr)
ddply(df, .(group), function(x){
  x[sample(nrow(x), 1), ]
})

Best regards,

Thierry

----------------------------------------------------------------------------
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

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