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

From   Richard Williams <>
Subject   Re: st: Understanding Factor variables - is order significant ?
Date   Tue, 25 May 2010 22:43:14 -0500

At 09:19 PM 5/25/2010, Michael N. Mitchell wrote:
Dear Richard

  I think I need to use my glasses!

Yes, Richard, you are exactly on target. It relates to the use of -#- instead of -##- .

My previous answer is still true, in the sense that when you do a#b and b#a, that you get a different reference "cell", and thus it is re-scrambling the coding.

I agree with everything you say. But, it still isn't clear to me why it should make a difference whether you use versus Tweaking your example, the following yield identical or equivalent results for regress but not for poisson:

sysuse auto, clear
generate bigtrunk = trunk > 15
generate biglen = length > 190
regress mpg bigtrunk#biglen
regress mpg b1.bigtrunk#b0.biglen
regress mpg b0.biglen#b1.bigtrunk
poisson mpg bigtrunk#biglen, nolog
poisson mpg b1.bigtrunk#b0.biglen, nolog
poisson mpg b0.biglen#b1.bigtrunk, nolog

Use of ## in the last 2 commands avoids the problem, but why is there a problem in the first place?

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

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