Using egen functions with replace

Question

I created a new variable from the mean of another variable using egen:

egen afd_lr2 = mean(afd_lire2w) if ost == 0

Now I would like to replace the values with the mean of another variable if ost == 1:

replace afd_lr2 = mean(afd_lireo) if ost ==1

This is not possible, as the mean function cannot be used with the replace command.

How can I achieve my goal?

Answer 1

This should work

egen afd_lr2 = mean(cond(ost == 0, afd_lire2w, cond(ost == 1, afd_lireo, .))), by(ost) 

Here is a test:

clear
input float(group y1 y2)
1 42   .
1 42   .
2  . 999
2  . 999
end

egen mean = mean(cond(group == 1, y1, cond(group == 2, y2, .))), by(group)

tabdisp group, c(mean)

----------------------
    group |       mean
----------+-----------
        1 |         42
        2 |        999
----------------------

The key is that the mean() function of egen feeds on an expression, which can be more complicated than a single variable name. That said, this is trickier than I would generally advise, as

generate work = afd_lire2w if ost == 0
replace work = afd_lireo if ost == 1
egen mean = mean(work), by(ost)

is easier to understand and should occur to a programmer any way.