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st: RE: No need for loops?


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: No need for loops?
Date   Thu, 4 Dec 2003 12:21:29 -0000

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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