st: calculate the standard deviation for a list of lagged values

[Larry Sen <larrysen78@gmail.com>]

Hi statalist,

I have a dataset about the firm's sales values in each year. The
structure of my dataset looks like this:

YEAR    FIRM_ID    SALES
1975        01                100
1976        01                200
1977        01                120
1978        01                130
1977        02                130
1978        02                150
1979        02                160
1980        02                180
1972        03                110
1973        03                130
1974        03                120
1975        03                190


For each observation, I want to calculate the stand deviation of the
sales using the previous ten years sales number. How can I do that?

I tried to use by firm_id: eegen sales_sd=std(sales) in[_n-11]/[_n-1]

But its not working :(.

Thank you!
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