ZEROMODEL
dependent-variable
zero-inflated-regressors < / options > ;
The ZEROMODEL statement is required if either ZIP or ZINB is specified in the DIST= option in the MODEL statement. If ZIP or ZINB is specified, then the ZEROMODEL statement must follow the MODEL statement. The dependent variable in the ZEROMODEL statement must be the same as the dependent variable in the MODEL statement.
The zero-inflated (ZI) regressors appear in the equation that determines the probability () of a zero count. Each of these
variables has a parameter to be estimated in the regression. For example, let
be the
th observation’s
vector of values of the
ZI explanatory variables (
is set to 1 for the intercept term). Then
is a function of
, where
is the
vector of parameters to be estimated. (The zero-inflated intercept is
; the coefficients for the
zero-inflated covariates are
.) If q is equal to 0 (no ZI explanatory variables are provided), then only the intercept term
is estimated. The “Parameter Estimates” table in the displayed output shows the estimates for the ZI intercept and ZI explanatory variables; they are labeled with
the prefix “Inf_”. For example, the ZI intercept is labeled “Inf_intercept”. If you specify
Age
(a variable in your data set) as a ZI explanatory variable, then the “Parameter Estimates” table labels the corresponding parameter estimate “Inf_Age”.
You can specify the following options in the ZEROMODEL statement after a slash (/):