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Re: st: RE: AW: AW: Problems with %tw when date() is used to create a date variable


From   vwiggins@stata.com (Vince Wiggins, StataCorp)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: AW: AW: Problems with %tw when date() is used to create a date variable
Date   Fri, 06 Nov 2009 12:27:08 -0600

Joachim Landstrom <joachim.landstrom@fek.uu.se> has been asking about handling
weekly data in Stata and is having particular trouble with the fact that Stata
enforces 52 weeks in every year.

Stata weeks are invertible and we don't want to lose that property.  If we
just counted weeks from 1960w1, then you could have 53 weeks in a year, but
you couldn't know what year in which a week occurs!  Not only that, but you
would need to tell Stata what day you would like weeks to start on.  We took
the equally awkward solution that weeks restart in each year, but now you get
weeks that are not all 7 days long (both the beginning and ending week in most
years).  At least these weeks are invertible with other time units.

If Joachim has daily data he observes only once per week (say every Monday),
then he can leave his time unit as daily, but tell Stata that they are
observed only every 7 days.

     . tsset mydate , daily delta(7)

Even if Joachim's dates are not all observed on the same day, so long as they
are observed only once in each week, it is not too difficult create a time
variable where we pretend they are observed on the same day each week.
Typing,

	. gen long wkdate = floor(mydate/7) * 7 + 3

sets the dates to their prior Monday.

The -delta(7)- in 

	. tsset wkdate, daily delta(7)

then tells Stata that the delta between dates is 7 days.  So, for example, if
we type,

	. gen lagx = L.x

each value of lagx is the the value from the previous Monday -- a 7 day lag.

All of Stata's time-series operations will enforce the the 7-day delta.


-- Vince 
   vwiggins@stata.com

 
 
------------ Original message ---------------------------
 
Subject: st: AW: AW: Problems with %tw when date() is used to create a date
variable
Sender: owner-statalist@hsphsun2.harvard.edu

How nice Martin! I does indeed solve my problem, but raises another
concerning years having 53 weeks.

When I convert my date values into weekly values as you proposed I end up
having duplicates of week 52 for those years that have 53 weeks (such as
e.g., 1980, 1985, 1991, 1996, 2002, and 2006). The Stata week values also
appear in duplicates so it is not just an issue of formatting. 

I presume this will pose a problem for me when I tsset the data and start to
run regressions on lagged values.

What to do with the duplicates (a side issue is of course why does not Stata
recognize the 53 week years)? Is there a workaround if needed (my dataset is
really on weekly basis so there is no option to run the analysis on daily
basis. I could of course run the analysis on monthly basis but I will then
loose too much information and hence it is also out of the question. . .

/Joachim 

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Martin Weiss
Sent: Friday, November 06, 2009 10:41 AM
Subject: st: AW: AW: Problems with %tw when date() is used to create a date
variable


<> 

See the conversion in action in the last line...

*************
clear*

inp str10 dates
"12/31/1974"
end

compress
 
gen myDate = date(dates, "MDY")
format myDate %td

list, noobs 

//wrong
di %tw myDate
//correct
di %tw wofd(myDate)
*************



HTH
Martin


-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Martin Weiss
Gesendet: Freitag, 6. November 2009 10:37
An: statalist@hsphsun2.harvard.edu
Betreff: st: AW: Problems with %tw when date() is used to create a date
variable


<> 

Not a riddle! -format-ting does not change the underlying value of the date
variable, only how it is -display-ed. To switch from daily to weekly
frequency, you want to use the conversion functions, as in

*************
help dates_and_times, mark(converting)
*************



HTH
Martin


-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Joachim
Landström
Gesendet: Freitag, 6. November 2009 10:37
An: statalist@hsphsun2.harvard.edu
Betreff: st: Problems with %tw when date() is used to create a date variable

Hi all,
I have encountered a strange problem that I do not know how to solve of if
there is a bug somewhere. The problem is as follows. From a string value
(such as e.g., 12/31/1974) I create myDate using date. The string value
12/31/1974 thus becomes myDate value 5478. Finally I format the myDate value
as e.g., %td or %tw. 

When I compare myDate with the original string I find a one-to-one
correspondance such that the myDate shows the same date as do the original
string (i.e., when the original string is 12/31/1974, the myDate %td format
is 31dec1974). However when I instead format myDate as %tw I find a
completely different result. E.g., the example 12/31/1974 then becomes
2065w19. . . Something must be very wrong and I appreciate any suggestion to
solve this riddle.

The code is as follows.

generate myDate = date(dates, "MDY")
format myDate %td

I have also tested to use mdy() to create myDate (which takes a few more
lines of code) and find identical results.

Regards
Joachim
___________________________________________
Joachim Landstrom
Assistant Professor
Department of Business Studies
Uppsala University, Sweden

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