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Re: st: Question about interactions


From   K Jensen <k.x.jensen@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Question about interactions
Date   Mon, 4 Mar 2013 16:30:03 +0000

Sorry to try your patience but can I test my understanding here?

The results for 1-2 (say) are calculated from the main effect betas
for variable1=1 and variable2=2?

Thankyou
Karin

On 4 March 2013 15:37, Maarten Buis <maartenlbuis@gmail.com> wrote:
> On Mon, Mar 4, 2013 at 3:48 PM, K Jensen wrote:
>> I am fitting two four-point scales as predictors in a logistic
>> regression and am interested in looking at the interaction between the
>> two.
>>
>> If I fit variable1##variable2 I get odds ratios for the following:
>> variable1
>>         2
>>         3
>>         4
>> variable2
>>         2
>>         3
>>         4
>> variable1#
>> variable2
>> 2       2
>> 2       3
>> 2       4
>> 3       2
>> 3       3
>> 3       4
>> 4       2
>> 4       3
>> 4       4
>>
>> I obviously don't understand this because it doesn't include cells
>> 1-2, 1-3, 1-4, 2-1, 3-1 or 4-1. Can someone explain this?
>
> For both variables the category 1 is the reference category and is
> thus, together with its interaction terms, excluded. More on this, and
> alternatives, can be seen here: M.L. Buis (2012) "Stata tip 106: With
> or without reference", The Stata Journal, 12(1), pp. 162-164.
> <http://www.maartenbuis.nl/publications/ref_cat.html>.
>
> -- Maarten
>
> ---------------------------------
> Maarten L. Buis
> WZB
> Reichpietschufer 50
> 10785 Berlin
> Germany
>
> http://www.maartenbuis.nl
> ---------------------------------
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