Logistic Interaction question

[Scott Carl]

Hi,

In using SPSS logistic regression, my categorical cross products have
the ‘wrong’ degrees of freedom. I have two binary independent variables,
say gender and race, and when I cross them I get 1 df. If I concatenate
them into one variable with four levels, I get what I consider the proper
df of 3 (n-1). Any assistance with grammar would be appreciated.

Scott Carl

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[Burleson,Joseph A.]

Scott:

While the resultant 4-level variable may correctly be viewed as having 3 df, the antecedent 2 binary predictor variables have 1 df each, and the interaction of them has, properly, 1 df, for a total of 3 df.

Hence, the output is not wrong.

Joe Burleson

-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Scott Carl
Sent: Friday, October 03, 2008 11:21 AM
To: [hidden email]
Subject: Logistic Interaction question

Hi,

In using SPSS logistic regression, my categorical cross products have
the ‘wrong’ degrees of freedom. I have two binary independent variables,
say gender and race, and when I cross them I get 1 df. If I concatenate
them into one variable with four levels, I get what I consider the proper
df of 3 (n-1). Any assistance with grammar would be appreciated.

Scott Carl

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[Scott Carl]

I am certain it is operator error. I see, in order to get what I am used
to a specify A B A*B, where I am used to specifying just A*B.

Thank you Joe.

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[Swank, Paul R]

An interaction of two dichotomous variables should have one df. The three df confound the main effects with the interaction.

Paul R. Swank, Ph.D
Professor and Director of Research
Children's Learning Institute
University of Texas Health Science Center
Houston, TX 77038


-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Scott Carl
Sent: Friday, October 03, 2008 10:21 AM
To: [hidden email]
Subject: Logistic Interaction question

Hi,

In using SPSS logistic regression, my categorical cross products have
the ‘wrong’ degrees of freedom. I have two binary independent variables,
say gender and race, and when I cross them I get 1 df. If I concatenate
them into one variable with four levels, I get what I consider the proper
df of 3 (n-1). Any assistance with grammar would be appreciated.

Scott Carl

=====================
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