Re: st: Question about interactions

[Maarten Buis <maartenlbuis@gmail.com>]

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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