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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 > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Stata date format %tw: how long is the weekend in a 9-day week?***From:*Sergiy Radyakin <serjradyakin@gmail.com>

**st: RE: Stata date format %tw: how long is the weekend in a 9-day week?***From:*Nick Cox <n.j.cox@durham.ac.uk>

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