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RE: st: Understanding Factor variables - is order significant ?


From   "Jesper Lindhardsen" <JESLIN01@geh.regionh.dk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Understanding Factor variables - is order significant ?
Date   Thu, 27 May 2010 09:33:43 +0200

I'm very happy with for the first (out of uncountable occasions) Stata
is wrong !!!!
Do you get a "find a bug" T-shirt ;-)

I'm looking forward to a fix as I find the factor variable feature
extremely useful.
Jesper



Jesper Lindhardsen
MD, Ph.d. student
Department of Cardiovascular Research
Copenhagen University Hospital, Gentofte
Denmark



-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Richard
Williams
Sent: 27. maj 2010 00:45
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Understanding Factor variables - is order significant ?

You wonder how such an obvious problem could be missed.  :) Actually, 
I think this should be a strong contender for most esoteric bug of 
the year award! Thanks for fixing it.

At 03:17 PM 5/26/2010, Roberto G. Gutierrez, StataCorp wrote:
>Jesper (and those others who have contributed on this thread) have
discovered
>a bug in how factor-variable interactions are being parsed in Stata.
The
>specific conditions that trigger this are as follows:
>
>    1. You specify a simple interaction (a single # sign) between two
or more
>       factor variables.
>
>    2. The first variable in the interaction has the value zero as one
of
>       its categories.
>
>    3. The first specification in the interaction has a base level that
is
>       not the default of zero (the lowest level for the first
variable).
>
>    4. At least one of the remaining variables in the interaction has a
base
>       equal to the lowest-valued category for that variable, 
> whether expicitly
>       specified or taken as the default.
>
>    5. Almost all estimation commands are affected by this bug, with
-regress-
>       being one notable exception.
>
>When the above conditions occur, Stata is attempting to omit an extra
cell in
>the interaction.  Sometimes, the cell will be omitted altogether, other
times
>Stata will produce a coefficient for that cell, but missing standard
errors
>and confidence intervals.  Either way, the model fit is thrown off
because the
>cell's coefficient is not properly estimated.
>
>We will fix this in the next executable update, to be made available
soon.
>
>--Bobby                                         --Jeff
>rgutierrez@stata.com                            jpitblado@stata.com
>*
>*   For searches and help try:
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>*   http://www.ats.ucla.edu/stat/stata/

-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu
WWW:    http://www.nd.edu/~rwilliam

*
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*   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/


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