st: calculating average in Stata

[James Bernard <jamesstatalist@gmail.com>]

Hi all,

I am trying to calculate average of observations for a variable. This
is a panel data... grouped by firms observed over 20 years.

Now, I want to calculate average of past three years of a variable to
construct a new variable.

Firm                  Year                sales
--------           ------------              --------------
A                 1990                    100
A                  1991                    missing
A                  1992                    200
A                  1993
A                   1994
A                   1995


So i would like to create a new average_3_sales variable that take in
year 1993 the average of sales observation from year 1990 to 1992....
What Stata does is that it simply returns missing as 1991 observation
is missing. Is there any solution to this?

In my research it would be more reasonable to ignore the missing value
and calculate average of 100 and 200 which is 150. so the value of the
new variable would be 150 for year 1993.

I appreciate your help in advance

James
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