Hi list,

I want to generate a variable that equals the mean of the values of the first two nonmissing variables in a three-variable list (var1, var2, var3). At the same time, I also want to record which two of the three variables in my list have been used to generate the new variable (I do not need the variable name; the order of the variable is enough). How can I do that efficiently? Thank you very much!

Code:
webuse auto,clear

replace headroom=. if mpg==17
replace weight=. if mpg==22|mpg==17
//I did the above manipulation so that some observations have only one nonmissing value in the varlist to be considered below
//so that you cannot simply apply egen rowmean to fulfil my task

keep rep78 headroom weight
rename rep78 var1
rename headroom var2
rename weight var3
//how to generate a variable that equals the mean of the first two nonmissing variables in the varlist (var1, var2, var3)
//and record which two of the three variables in my list have been used to generate the new variable?