st: smart averaging

[Murat Mercan <mamercan@gmail.com>]

Hi,

I have the following problem I have been unable to decipher. I have the
following data set:
id      year       income
1       2006      100
1       2007      50
1       2008       .
1       2009       .
2       2006       50
2       2007       50
2       2008       50
2       2009       50
I want to average the income variable for those four years. I tried to use
the following command:

bys  id: egen average_inc = mean(income) if inrange( year,2006,2009)

However, that averages person 1's income, too. I need to average just
person 2's income.
Can anyone suggest how to do that?  Thank you in advance.
Murat
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