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Re: st: Count data interaction terms


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Count data interaction terms
Date   Tue, 23 Aug 2011 11:20:14 +0200

On Mon, Aug 22, 2011 at 5:50 PM, DA Gibson wrote:
> Im running an investigation into how unhealthy lifestyles (obestiy,
> smoking etc) effect health service usage and ideally id like to use a
> count analysis to do this. However im not sure whether as with OLS
> analyses you can include interaction terms e.g. y = B0
> +B1(X1)+B2(X2)+B3(X1*X2) where X1 and X2 are binary variables. Is this
> possible using count data?

Yes, the tricky part is interpretation, but that is not a big deal as
long as you interpret the incidence rate ratios rather than marginal
effects. Consider the example below. The baseline tells you that a
visitor to the park without camper, who visited the park alone and has
no kid with him/her is expected to catch .39 fish per visit(*). An
extra person added to the group increases that rate by a factor 3.07,
i.e. (3.07-1)*100%=207 % (the main effect of person). If this extra
person happens to a child, this effect of number of persons decreases
by a factor .78, i.e. -22%.

*------------------ begin example -----------------
webuse fish, clear
gen byte baseline = 1
poisson count i.camper c.persons##c.child baseline, ///
        irr nocons
*----------------- end example --------------------
(For more on examples I sent to the Statalist see:
http://www.maartenbuis.nl/example_faq )

Also see: M.L. Buis (2010) "Stata tip 87: Interpretation of
interactions in non-linear models", The Stata Journal, 10(2), pp.
305-308.

Hope this helps,
Maarten

(*) I still have Stata 11, so I need this trick with a "variable"
baseline which is secretly a constant. I understand that in Stata 12
the display of the baseline rates, odds, hazards, etc. are no longer
suppressed, so there this trick is no longer necessary.

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


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