I just saw different cases and I understand the _n < 12 part -- it's
just changing to missing if there's a program = 1 within the first 12
months of the sample. I thought it somehow turned to missing the
observations when program = 1 but there were missing values to compute
the average. I need to take this into account, but I guess I didn't
specify it before.
Thanks.
> -----Original Message-----
> From: de la Garza, Adrian
> Sent: Thursday, December 04, 2003 3:15 PM
> To: [email protected]
> Subject: st: No need for loops?
>
>
> 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:[email protected]]
> > Sent: Thursday, December 04, 2003 7:21 AM
> > To: [email protected]
> > 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
> > [email protected]
> >
> > 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: [email protected]
> > > 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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> >
>
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