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From |
K Jensen <k.x.jensen@gmail.com> |

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 > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/faqs/resources/statalist-faq/ > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Question about interactions***From:*K Jensen <k.x.jensen@gmail.com>

**References**:**st: Question about interactions***From:*K Jensen <k.x.jensen@gmail.com>

**Re: st: Question about interactions***From:*Maarten Buis <maartenlbuis@gmail.com>

**Re: st: Question about interactions***From:*K Jensen <k.x.jensen@gmail.com>

**Re: st: Question about interactions***From:*David Hoaglin <dchoaglin@gmail.com>

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