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st: RE: No need for loops?


From   "de la Garza, Adrian" <ADelagarza@imf.org>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: No need for loops?
Date   Thu, 4 Dec 2003 15:14:32 -0500

Nick,

This is very insightful. I don't understand, however, what the _n<12
does. Can you please clarify? I don't know why, for instance, if for a
country my -ratio- variable is not populated (all missing values) but
-program- does have 1s now and then, why -prev12- gets a missing value
when -program- = 1 but -next12- gets a 0 (when it should be missing,
too)?

Also, below you wrote:

> by country: replace next12 = . if pronext12 | _n < 12 

This should be 
 
  by country: replace next12 = . if pronext12 > 0 | _n < 12

right?


Thanks again.
Adrian
 
> -----Original Message-----
> From: Nick Cox [mailto:n.j.cox@durham.ac.uk] 
> Sent: Thursday, December 04, 2003 7:21 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: No need for loops?
> 
> 
> I hope I don't get to regret making that assertion, 
> which as said was about "most" cases. However, I didn't say 
> that I would be able to spot such a solution 
> myself! 
> 
> In this case, progress is indeed possible, bringing 
> in also an idea from Michael Blasnik on getting 
> the sum of the previous # observations, without 
> any loops. 
> 
> The average of the previous 12 is the 
> difference between two cumulative sums, divided 
> by 12 
> 
> bysort country (date) : 
> 	gen prev12 = (sum(ratio[_n-1]) - sum(ratio[_n-13])) / 12
> 
> However, we only want that if -program- is equal to 1 or 
> (presumably) it is in fact an average based on 12 measurements. 
> 
> by country : replace prev12 = . if program != 1 | _n < 12 
> 
> Looking ahead in time seems trickier until you see that 
> we just need to reverse time (no science fiction or bizarre 
> physics involved, just negation): 
> 
> gen ndate = -date 
> 
> Then everything is just the same, modulo _including_ 
> the month of a program,  
> 
> bysort country (ndate) : 
> 	gen next12 = (sum(ratio) - sum(ratio[_n-12])) / 12
> 
> except that we want to keep track of any program in the next
> 11 [sic] months. We just look out for a nonzero sum of -program- 
> 
> by country : 
> 	gen pronext12 = sum(program[_n-1]) - sum(program[_n-12]) 
> by country: replace next12 = . if pronext12 | _n < 12 
> 
> I guess that Adrian's other variables are calculated
> by variations on this theme. 
> 
> sort country date
> 
> Some things could be done differently if you have -tsset- 
> the data. This code assumes no gaps within panels. 
> 
> The device of reversed time is exemplified further 
> at 
> 
> http://www.stata.com/support/faqs/data/dropmiss.html
> 
> http://www.stata.com/support/faqs/data/missing.html
> 
> Nick 
> n.j.cox@durham.ac.uk 
> 
> P.S. There is a quite different way to approach 
> some of this using the program -tsspell- from SSC. In 
> this you define a spell as starting with 
> -program- being 1 and then block the subsequent 
> months making use of a created variable _seq. 
> 
> de la Garza, Adrian
> > Sent: 04 December 2003 04:10
> > To: statalist@hsphsun2.harvard.edu
> > Subject: st: No need for loops?
> > 
> > 
> > I am trying to learn from Nick Cox's suggestion that no 
> > loops are needed in most cases for data management. 
> > However, I can't think of a way to do the following without a loop.
> >  
> > I have the next database in panel format and I need to take 
> > averages within countries when my dummy variable 'program' 
> > indicates it. Basically, I need to generate a variable that 
> > computes the 12-month average for the observations 
> > immediatly prior to a 1 in 'program'. 
> >  
> > Take Brazil in 1983m3, when program = 1. I want my 
> > generated variable, let's call it ratio_before, to get the 
> > value of the 12-month average right before 1983m3, and put 
> > this value in the same line where program = 1. I know this 
> > average should be 0.00639. 
> >  
> > Then, I also want to take averages for the subsequent 
> > 12-months (including the month when program = 1), for the 
> > 12-month period after that, etc. These are always annual 
> > (12-month) averages for the first year when the program is 
> > implemented as indicated by the 1 in the dummy, then for 
> > the 2nd year, for the 3rd year, etc., and we can call these 
> > variables ratio_1y, ratio2y, ratio3y, ratio4y, and ratio5y. 
> > All of these computed averages should go in the same line 
> > where program = 1.
> >  
> > The only thing is: I should stop taking the averages for 
> > the subsequent years if a new program is implemented (i.e., 
> > if I find another 1 in my 'program' variable within the 
> > period for the average computed).
> >  
> > I would very much appreciate any insight on this dull data 
> > management procedure.
> > Thanks.
> >  
> > date      country     ratio                program
> > 1982m1	 bra	 0.001943	 0	
> > 1982m2	 bra	 0.003863	 0	
> 
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