The YVARIATE command defines the response variable which is to be used as the
y-variate in subsequent model fitting. For binary models the y-variate must
itself be binary (0,1), while for Poisson models it must be non-negative
integer. There is no default to this command; it must be used before model
fitting can commence. The current y-variate may be obtained by issuing the
DISPLAY VARIABLES command.