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