Take randomly sample based on groups

Question

I have a df made by almost 50,000 rows spread in 15 different IDs (every ID has thousands of observations). df looks like:

        ID  Year    Temp    ph
1           


        

Answer 1

mydata1 is your original data(not tested)

mydata2<- split(mydata1,mydata1$ID)
names(mydata2)<-paste0("mydata2",1:length(levels(ID))) 
mysample<-Map(function(x) x[sample((1:nrow(x)),size=500,replace=FALSE),], mydata2)

library(plyr)# for rbinding the mysample
ldply(mysample)