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Date:   Fri, 2 Jul 2004 08:49:47 +0200
Reply-To:   Marta García-Granero <biostatistics@terra.es>
Sender:   "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:   Marta García-Granero <biostatistics@terra.es>
Organization:   Asesoría Bioestadística
Subject:   Re: LOGISTIC REGRESSION: VARIABLES=...BY [var]
In-Reply-To:   <5.1.0.14.2.20040701145307.00a4bde0@pop.mindspring.com>
Content-Type:   text/plain; charset=ISO-8859-1

Hi Richard,

RR> From the syntax manual entry for LOGISTIC REGRESSION, syntax template:

RR> LOGISTIC REGRESSION [VARIABLES =] dependent var RR> [WITH independent varlist [BY var [BY var] ... ]]

RR> "· You can indicate an interaction term on the variable list by using the RR> keyword BY to separate the individual variables."

RR> I don't quite understand this. If you use "BY var", does that mean RR> interaction terms between "var" and all preceding variables, or what? Or RR> can specific interaction variables be included, or excluded, on VARIABLES RR> or METHOD subcommands?

THe syntax defines specific interaction terms. For example:

LOGISTIC REGRESSION VAR=kyphosis WITH age starvert age BY starvert /CRITERIA = PIN(.05) POUT(.10) ITERATE(20) CUT(.5) .

This syntax models the dependent variable Kyphosis as a function of:

- Age - Starvert - The interaction between the two above

LOGISTIC REGRESSION VAR=kyphosis WITH age numvert starvert age BY starvert /CRITERIA = PIN(.05) POUT(.10) ITERATE(20) CUT(.5) .

This syntax models the dependent variable Kyphosis as a function of:

- Age - Numvert - Starvert - The interaction between age & starvert (numvert is not involved)

I always use "*" to indicate interaction terms, it looks clearer to me:

LOGISTIC REGRESSION VAR=kyphosis WITH age numvert starvert age*starvert /CRITERIA = PIN(.05) POUT(.10) ITERATE(20) CUT(.5) .

-- HTH, Marta mailto:biostatistics@terra.es


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