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Re: st: RE: Stata date format %tw: how long is the weekend in a 9-day week?


From   Sergiy Radyakin <serjradyakin@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Stata date format %tw: how long is the weekend in a 9-day week?
Date   Fri, 29 Jul 2011 15:53:47 -0400

Dear Nick, thank you very much for the reference, that indeed answers all the
questions that I had regarding the %tw format.
Best, Sergiy

On Fri, Jul 29, 2011 at 12:00 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> Please see
>
> SJ-10-4 dm0052  . . . . . . . . . . . . . . . . Stata tip 68: Week assumptions
>        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>        Q4/10   SJ 10(4):682--685                                (no commands)
>        tip on Stata's solution for weeks and on how to set up
>        your own alternatives given different definitions of the
>        week
>
> For Stata, week 1 of any year starts on 1 January, regardless of what day it is.
>
> Week 52 may have (indeed will have) more than 7 days.
>
> These are unlikely to fit with many needs, and the Tip above is, in the absence of any apparent competition, all that is written on the subject. I agree, however, with your implication that this should all be better documented in the manual. The Stata 12 manuals do reference the Tip above.
>
> If we go back to basics, the facts are that weeks do not nest inside years or months and that practical definitions of week differ a lot, so Stata is agnostic on the point.
>
> Nick
> n.j.cox@durham.ac.uk
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Sergiy Radyakin
> Sent: 29 July 2011 16:50
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Stata date format %tw: how long is the weekend in a 9-day week?
>
> Dear All,
>
> I have a question regarding the Stata's %tw format. The manual (help
> dates_and_times) says in this format the numeric values are
> interpreted as:
>
> "a %tw value records the number of weeks from the first week of 1960"
>
> I have the following questions:
>
> 1) The year 1960 started on a Friday. Why is Wednesday the 6th of Jan
> 1960 is still considered the first week of 1960?
>
> See the following code, which shows the day-to-week assignment in the
> month of January 1960:
> forval i=0/30 {
>  display wofd(`i')
> }
>
> Reformulating the question is the week reported in the format like 1975w9
>  A) the calendar week?
>  B) the number of the 7-day-long group since the beginning of the
> year where a particular date belongs to?
>  C) something else?
>
>
> 2) The second question comes as we depart from 1960 and move on to the
> current dates. For an example take a value like 1975. The date that is
> 1975 weeks away from the Jan 1, 1960 falls somewhere in early November
> 1997, while Stata reports it as the 52 week of the said year. I can
> substantiate the question with the example in Excel:
>
> Format a cell with the default date format. Enter the value 21916,
> which corresponds to Jan 1, 1960 (in the US notation 1/1/1960)
> Format another cell with the default date format. Enter a formula
> =A1+7*1975 - which will add 1975 weeks (of 7 days each) to the first
> date. The result (in the US notation) should be 11/7/1997.
>
>
> 3) My final question is related to the length of the year in weeks
> assumed by Stata: I have never seen Stata reporting the value YYYYw53
> in any of the years, which is rather strange, since any year will
> contain dates beyond the 52 week. The investigation with the following
> code led to very interesting results:
>
> forval i=0/365 {
>  display `i' " " wofd(`i')
> }
>
> Here 0-365 are the 366 days of the good old year 1960 which was a leap
> year. Interestingly, according to Stata the year ended in a long week
> consisting of 9 days!!!
> 357 51
> 358 51
> 359 51
> 360 51
> 361 51
> 362 51
> 363 51
> 364 51
> 365 51
> (I just hope there was a very-long weekend :)
>
> Jan 1, 1961 (day 366) is then assigned week 52 and it lasts for 7 days as usual.
>
> The manual declares the valid range for week number 1-52, which agrees
> with the results above, but doesn't give any insight into the fact
> which week of the year will have 8 or 9 days.
>
> I don't think there is a bug in Stata with regards to the handling of
> dates (though the last issue illustrates an apparent unorthodox
> approach), but rather I seek a more elaborate explanation of how Stata
> comes to the (formatted) values like "1997w52" for value 1975 and the
> format %tw. The manual for date-time handling is rather cumbersome and
> is more elaborate about atomic time and leap seconds rather then 9-day
> weeks! Perhaps it can be improved by an easy "getting started" page
> that would deal with only one (recommended) format for date and the
> rest is put into a different reference-style topic.
>
> The version of Stata does not play a role here and the quoted results
> are reproducible in Stata v9-11 (Windows).
>
> Thank you,
>    Sergiy Radyakin
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