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(This article was first published on R HEAD, and kindly contributed to R-bloggers)

egen(stata cmd) compute a summary statistics by groups and store it in to a new variable. For example, the data has three variables, id, time and y, we want to compute the mean of y by for each id and then store it as a new variable mean_y.

In stata, the command would be

egen mean_y = mean(y), by(id)

In R, this task can be completed by ave

Generate dataset:

id <- rep(1:3,each=3)
t<-rep(1:3,3)
y<-sample(1:5,9,replace=T)
my_data<-data.frame(id=id,time=t,y=y)

The default summary statistics is mean. However, we can assign a particular function to compute the summary statistics. For example, if we want to compute the sd of y by id, then we can have

Remark: The within evaluate an expression in an environment created from the data.frame. In addition, it will modify the data.frame and return it back(in our case, it create new variables, mean_y or sd_y )

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