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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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: No need for loops?***From:*"de la Garza, Adrian" <ADelagarza@imf.org>

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