.Purpose
Specifies that geometric mixture models will be fit and
indicates the value of the mixing parameter .
Syntax
GMIX mixvalue
Arguments and Subcommands
mixvalue
The value of the mixing parameter 
to be used in fitting the
model.
Remarks
The geometric mixture model has the form:
where 
is either a relative risk term
(
) or an excess relative risk term (
). By default, terms in a geometric
mixture model are relative risk terms. The GTERM
command is used to change the term type.
The mixing parameter may have any value. A value of 1 corresponds to a generalized multiplicative model in which the risk function has the form:
If mixvalue equals 0, then the model to be fit is similar to a product additive model. This model has the form:
where is either a relative or excess
risk term.
Examples
a) To fit a model which allows for a multiplicative effect of amount smoked ( smk) on dose with no interaction. The background is also modeled as a loglinear function of the logarithm of attained age lage. This model can be written:
GMIX 1 @
LOGL 0 %CON lage @
LOGL 1 dose @
LOGL 2 smk @
FIT @
b) To fit an additive excess relative risk model to these same data. This model is
GMIX 0 @
LOGL 0 %CON lage @
LOGL 1 dose @
LOGL 2 smk @
FIT @
c) To fit a mixture model to the smoking and dose data with a mixing parameter of 0.9. It is assumed that the term-specification commands were given previously so only the GMIX and FIT commands are necessary to change the model. The new model is:
GMIX 0.9
FIT @