Date:         Sat, 14 Aug 2010 17:57:13 -0400
Reply-To:     R B <ryan.andrew.black@gmail.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         R B <ryan.andrew.black@gmail.com>
Subject:      Re: logistic regression interaction
In-Reply-To:  <1281819852709-2635437.post@n5.nabble.com>
Content-Type: text/plain; charset=ISO-8859-1

Anna,

To answer your question, we need to write out the equation, and to
write out the equation we need to have common language:

If you have two categorical independent variables (Factor A has two
levels and Factor B has 3 levels), then let A2 be the indicator of
Factor A and B2 and B3 be the indicators of Factor B. The first levels
of Factor A and B are the reference groups. Lower case letters denote
coefficients.

Equation:

logit(y) = m
           + a2*A2
           + b2*B2
           + b3*B3
           + ab22*A2*B2
           + ab23*A2*B3

Then,

Odds|(A=1,B=1) = exp(m)
Odds|(A=1,B=2) = exp(m + b2)
Odds|(A=1,B=3) = exp(m + b3)
Odds|(A=2,B=2) = exp(m + a2 + b2 + ab22)
Odds|(A=2,B=3) = exp(m + a2 + b3 + ab23)

Ryan

On Sat, Aug 14, 2010 at 5:04 PM, annastella <stellakos@gmail.com> wrote:
> Hi Ryan,
>
> Thanks a lot for that-will have a go with it and see what it gives me.
>
> However, may I ask if you or anyone knows a way to calculate the odds ratios
> (not the CIs necessarily) for the categories missing in the output (i.e. 3
> social class groups for non shift-workers)? Not in terms of syntax really,
> just want to see I understand the interaction calculation right while
> working this out.
>
> Many thanks
> Anna
> --
> View this message in context: http://spssx-discussion.1045642.n5.nabble.com/logistic-regression-interaction-tp2635146p2635437.html
> Sent from the SPSSX Discussion mailing list archive at Nabble.com.
>
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