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# Re: st: Question about interactions

 From K Jensen To statalist@hsphsun2.harvard.edu Subject Re: st: Question about interactions Date Tue, 5 Mar 2013 10:15:35 +0000

```Thanks--that was very helpful.

Best wishes
Karin

On 4 March 2013 16:55, David Hoaglin <dchoaglin@gmail.com> wrote:
> Karin,
>
> Together, the two 4-point scales would yield a total of 16 predictors
> (each of which is an indicator variable for a combination of a
> category of variable1 and a category of variable2).
>
> In your model the combination of variable1=1 and variable2=1
> corresponds to the constant term.
>
> The combination of variable1=1 and variable2=2 corresponds to the
> "main effect" for variable2=2 (and similarly for variable2=3 and
> variable2=4).
>
> The combination of variable1=2 and variable2=1 corresponds to the
> "main effect" for variable1=2 (and similarly for variable1=3 and
> variable1=4).
>
> The coefficients for the other 9 combinations of a category of
> variable1 and a category of variable2 appear under variable1#variable2
> in the sketch of output that you included.
>
> I hope this is helpful.
>
> David Hoaglin
>
> On Mon, Mar 4, 2013 at 11:30 AM, K Jensen <k.x.jensen@gmail.com> wrote:
>> 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
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```