Expanding on a question on Stack Overflow I’ll show how to make a stratified random sample of a certain size:

d <- expand.grid(id = 1:35000, stratum = letters[1:10])

p = 0.1

dsample <- data.frame()

system.time(

for(i in levels(d$stratum)) {

dsub <- subset(d, d$stratum == i)

B = ceiling(nrow(dsub) * p)

dsub <- dsub[sample(1:nrow(dsub), B), ]

dsample <- rbind(dsample, dsub)

}

)

# size per stratum in resulting df is 10 % of original size:

table(dsample$stratum)

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**Tags:** R, Stratified Random Sampling