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st: RE: question about calculating discounted sum


From   "Scott Merryman" <smerryman@kc.rr.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: question about calculating discounted sum
Date   Sun, 20 Feb 2005 16:37:04 -0600

How about attacking this problem directly:

bysort firm (year): gen num = _n
gen sum = 0
by firm: replace sum = income + income[1]/num[2] if num== 2
by firm: replace sum = income + income[1]/num[3] + income[2]/num[2] /// 
	if num ==  3
by firm: replace sum = income + income[1]/num[4] + income[2]/num[3] + ///
	income[3]/num[2] if num ==  4
by firm: replace sum = income + income[1]/num[5] + income[2]/num[4] + /// 
	income[3]/num[3] + income[4]/num[2] if num == 5
by firm: replace sum = income + income[1]/num[6] + income[2]/num[5] + ///
income[3]/num[4] + income[4]/num[3] + income[5]/num[2] if num == 6

Hope this helps,

Scott

P.S.  `=seq' is the same as seq[1] or 1

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Robin Luo
> Sent: Saturday, February 19, 2005 1:18 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: question about calculating discounted sum
> 
> I need to calculate discounted sum for a variable in a panel data. My
> data is like:
> 
>  FirmID      year     income
>    100        1980     50000
>    100        1981     51000
>    100        1982     54000
>    100        1983     60000
>    100        1984     59000
>    100        1985     62000
>    101        1970     18000
>    101        1971     18500
>    101        1972     20000
>    101        1973     24000
> 
> Basically, the data is a panel data, grouped by "FirmID", with
> hundreds of frims. What I need to do is to calculate a moving
> discounted sum of income for each firm-year. That is, calculate a sum
> of income from a firm's first year to the current year, but each
> year's income will be discounted by that year's distance to the
> current year and the discount rate is the inverse of the distance
> "1/(current year - year +1)". For example, for firm 100 at 1982, this
> calculation would be like: 54000 + 51000*1/2 +50000*1/3, while for
> firm 100 at 1984 it would be like 59000 + 60000*1/2 + 54000*1/3 +
> 51000*1/4 + 50000*1/5. I tried the following program:
> 
> by FirmID: gen seq=_n
> 
> gen suminc=0
> 
> quietly forval i=1/`=seq' {
>            by FirmID: replace suminc = suminc + income[`i']/(`=seq' - `i'
> +1)
>            }
> 
> But problems are: 1) the loop seems not allowing "by" command; 2) the
> loop does not really launch, because even after I get rid of "by
> FirmID" the loop seems to go only one cycle instead of "_n" cycles.
> 
> Many many thanks for any help!
> 
> Robin Luo



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