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)

Orignal data:

> my_data
  id time y
1  1    1 4
2  1    2 1
3  1    3 4
4  2    1 2
5  2    2 3
6  2    3 3
7  3    1 4
8  3    2 4
9  3    3 3


> within(my_data, {mean_y = ave(y,id)} )
  id time y   mean_y
1  1    1 4 3.000000
2  1    2 1 3.000000
3  1    3 4 3.000000
4  2    1 2 2.666667
5  2    2 3 2.666667
6  2    3 3 2.666667
7  3    1 4 3.666667
8  3    2 4 3.666667
9  3    3 3 3.666667

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

within(my_data, {sd_y = ave(y,id,FUN=sd)} )
  id time y      sd_y
1  1    1 4 1.7320508
2  1    2 1 1.7320508
3  1    3 4 1.7320508
4  2    1 2 0.5773503
5  2    2 3 0.5773503
6  2    3 3 0.5773503
7  3    1 4 0.5773503
8  3    2 4 0.5773503
9  3    3 3 0.5773503

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 )