Friday, February 24, 2006

st: RE: RE: xi command deletes variables without warning

Two more approaches to this that Nick hasn't mentioned...

1. Use the -prefix()- option to specify a different prefix each time you use -xi-. When -xi- is run it deletes any existing variables with the same prefix (usually "_I") that have previously been created by -xi-. For example:

xi, noomit prefix(_I) i.region i.income xi, noomit prefix(_J) i.interval

2. Clear the dataset characteristics where Stata stores the information on variables created by -xi-. For example:

xi, noomit i.region i.income char _dta[__xi__Vars__Prefix__] char _dta[__xi__Vars__To__Drop__] xi, noomit i.interval

Hope this helps

David

BTW (note for Stata tech people), there seems to be a bug in the -noomit- option... If -xi- is used with -noomit- it defaults to having no prefix rather than the usual "_I" and the dataset characteristics are set wrong, e.g.

. sysuse auto (1978 Automobile Data)

. xi, noomit i.rep78

. char li _dta[__xi__Vars__To__Drop__]: rep78_1 rep78_2 rep78_4 rep78_5

(rep78_3 has been created but is not listed as a variable to drop)

-----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox Sent: 24 February 2006 08:44 To: statalist@hsphsun2.harvard.edu Subject: st: RE: xi command deletes variables without warning

Everyone!

In 18th century landscape architecture in Britain, designers used features called "ha-has" in country estates. These were little grassy trenches into which walkers dropped without warning. As some wit pointed out at the time, the "ha-ha" surprise or shock was never the same the second time round.

-xi:- does do exactly what you report. It is a deliberate, designed feature. The argument is that

(a) It is all too easy otherwise to accumulate lots and lots and lots of dummy variables from even a few analyses. In a strong sense, such variables contain no new information, as they can be derived immediately from the original variables. For most purposes, such variables just bloat datasets unnecessarily.

(b) It is very easy to generate them all over again with a subsequent -xi:-.

(c) If you do want to keep those dummies, it is easy to generate them otherwise or to -rename- those you want.

Nick n.j.cox@durham.ac.uk

Thuy Le > I have a panel data set with about 20 variables and less than 700 > observations. I used the following command in STATA9 SE > > .xi, noomit i.region i.income > > to generate the dummies and it work fine. I saved the file and when I > used the command again with the same command for another variable > > .xi, noomit i.interval > > STATA somehow deleted some of the previously generated dummies. > > I used another command with no problem > > .qui tab, gen(interval_) > > Is there anyone experienced the same problem?

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