Bookmark and Share

Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.


[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: st: Question about correlated variables in zinb regression


From   Maarten buis <[email protected]>
To   [email protected]
Subject   Re: st: Question about correlated variables in zinb regression
Date   Fri, 11 Mar 2011 16:16:55 +0000 (GMT)

--- On Fri, 11/3/11, rachel grant wrote:
> I am modelling counts of amphibians arriving at a breeding
> site with zinb regression. I am using two variables to
> predict zeros. These two are highly correlated with each
> other (0.8 correlation coefficient) as one is derived 
> partially from the other. If I include them both in the
> model one coefficent is negative and one becomes positive!
> it is not really possible that there can be a positive
> correlation between this variable and my zero counts, it
> makes no sense. 

The key is that by adding both to your model you are looking
at the effect of one while keeping the other constant. Think
of your effects as comparing two observations that are equal
with respect to your first variable but differ with respect
to your second variable: what difference in zero count would
you expect in that case.

Hope this helps,
Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://www.maartenbuis.nl
--------------------------


      

*
*   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/


© Copyright 1996–2018 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   Site index