tapply for aggregate statistics

The use of apply function can avoid a for loop and it leads to faster computing. This example is about how to use tapply to compute some group/aggregated statistics.

Use tapply for most basic by processing (By processing = Aggregate statistics = stratified estimates; statistics computedafter cross-classifying data)

> y <- 1:8
> sex <-c(rep(’male’,4),rep(’female’,4))
> treat <-rep(c(’A’,’B’),4)
> sex
[1] "male" "male" "male" "male" "female" "female" "female" "female"
> treat
[1] "A" "B" "A" "B" "A" "B" "A" "B"
> tapply(y, sex, mean)
female male
6.5 2.5
> tapply(y, treat, mean)
A B
4 5
> tapply(y, list(sex,treat), mean)
A B
female 6 7
male 2 3