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st: RE: Creating variables = sum(last year)


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Creating variables = sum(last year)
Date   Wed, 10 Sep 2003 18:38:11 +0100

Adapting a Michael Blasnik dodge, we fill in 
all gaps and sum over (at most) the last 365 
days: 

bysort id (date) : gen diff = date[_n+1] - date
local N = _N 
expand diff 
gen byte original = _n <= `N' 
replace vol = 0 if !original 
bysort id date : replace date = date + _n + 1 
by id : gen X = sum(vol)
by id : replace X = X - max(0, X[_n-365])
keep if original  
drop original diff 

Notes: 

1. No special code for leap years. 
2. No checking for duplicate days within ids. 
I've not thought through whether that's 
a problem. 
3. It would be nice to do this without 
changing the data structure, even temporarily. 

Nick 
n.j.cox@durham.ac.uk 

> -----Original Message-----
Jitian Sheu

> My question is as following:
> 
> Suppose I have data as following:
> 
> obs    id     date                     vol            X
> 1        1     01 jan 1995           a               -
> 2        1     28 feb 1995             b            a
> 3        1     31 oct 1996             c            a
> 4        1     25 dec 1996             d            d
> 5        1     29 dec 1996             e            c+d
> 6        1     15 nov 1997             f             c+d
> 7        2     01 jan 1994             g            -
> 8        2     10 jan 1994             h            g
> 9        2     25 jan 1994             i            g+h
> 10      2     3 feb 1994                 j           g+h+i
> 11      2     28 feb 1994             k            g+h+i+j
> 12     2       01 jan 1995             l            g+h+i+j+k
> 13     2      11 jan 1995             m            g+h+i+j+k+l
> 
> 
> What I want is a new varialbe, say call X
> Where X is the summation of "VOL" for the same ID, but, the 
> summation only
> count for one year behind the date of underlying observation.
> 
> For example, take a look for observation #5.
> This observation has date ="29 Dec 1996" (for ID=1)
> The value of this new variable = all "VOL" occured between 
> "29 Dec 1995" -
> "28 Dec 1996".
> In this case the value is "c+d"
> 
> Take another example.
> Let's see the observation #10.
> This observation has date= "3 Feb 1994"
> I want the X to be the summation of all VOL occured between 
> "3 Feb 1993" to
> "2 Feb 1994", ===> =g+h+i

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