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Re: st: annual and 3years average


From   Neil Shephard <nshephard@nhs.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: annual and 3years average
Date   Wed, 17 Sep 2008 12:07:38 +0100

Fabian Brenner wrote:
Dear all,

I have several observations called "ROE" for the "years" (from 1979 to 2006). There is a different number of observations for each year.
My data look like this:
"year" "ROE" "Average" "threeyearaverage"

79 12 ? ? 79 9 ? ?
79 2 ? ?
80 3 ? ?
81 20 ? ?
81 5 ? ?
82 3 ? ?
82 6 ? ?
82 9 ? ?
82 8 ? ?
. . .
. . .
. . .

I want to compute the average of the observations for each year, e.g. for 1979: (12+9+2)/3 (I tried to sort the observations and to divide the sum by _n but it didn't work...)
bysort year : egen average = mean(ROE)
In a second step I want to get the average ROE for the past three years ("threeyearaverage") (beginning in 1982), e.g. for 1982 it should be the average of the ROE in 1979 plus the average ROE in 1980 plus average ROE in 1981, divided by 3.
Thats an inappropriate way of calculating the three year average, as it fails to account for the fact that there are different numbers observations from each year, thus the weights aren't equal. This is covered in most basic statistics books. You therefore have two options, a) use weights; b) use the raw data. Since -egen newvar = mean()- doesn't allow weights I'd be inclined to go with b).

You therefore need to generate a variable that bins your data...

gen year3 = .
replace year3 = 1 if(year >= 79 & year <= 82)
replace year3 = 2 if(year >= 83 & year <= 85)
replace year3 = 3 if(year >= 86 & year <= 88)
....
bysort year3 : egen threeyearaverage = mean(ROE)

Neil

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