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Re: st: Count data interaction terms


From   DA Gibson <DAGibson1@sheffield.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Count data interaction terms
Date   Tue, 23 Aug 2011 15:24:02 +0100

Thank you so much, this is brilliant!

On 23 August 2011 15:18, Maarten Buis <maartenlbuis@gmail.com> wrote:
> On Tue, Aug 23, 2011 at 3:54 PM, DA Gibson <DAGibson1@sheffield.ac.uk> wrote:
>> If i run a poisson analysis for number of doctor appointments with
>> independent dummy variables (equalling 1) for being obese and smoking
>> as well as an interaction term between them and get results showing
>> IRR values of, for example, .3(baseline) 1.4(obese), 2.3(smoker) and
>> 1.4(interaction).
>>
>> Then the interpretation is that an obese individual who doesnt smoke
>> is likely to visit the doctor ((1.4-1)*100%) 40% more times than their
>> non-obese counterpart. A non-obese individual who does smoke is likely
>> to visit the doctor ((2.3-1)*100%) 130% more times than their
>> non-smoker counterpart.
>
> That is correct
>
>> And the interaction term would suggest that an
>> individual who both smokes and is obese would visit the doctor
>> ((1.4-1)+(2.3-1)+(1.4-1)*100%) 210% more often than someone who isnt
>> obese and doesnt smoke?
>
> Not quite, obese and smoking respondents visits doctors 1.4*2.3*1.4 =
> 4.5 times (i.e. 350%) more often than their non-obese and non-smoking
> counterparts.
>
> In addition you can also look at the effect of being obese for smoking
> individuals, which is 1.4*1.4= 1.96. So if you are a smoker than being
> obese will lead to an 96% increase in the number of visits to the
> doctor. Similarly the effect smoking when being obese is 2.3*1.4=
> 3.22, i.e. smoking leads to 222% increase in the number of visits to
> doctors when a person is obese.
>
> Finally it can be useful to interpret the interaction directly. In
> this case being obese leads to a 40% increase in the effect of smoking
> (and, as interaction effects are symmetrical, smoking leads to a 40%
> increase in the effect of being obese)
>
> Hope this helps,
> Maarten
>
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
>
>
> http://www.maartenbuis.nl
> --------------------------
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