The COUNTREG Procedure |
ZEROMODEL Statement |
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 immediately after 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 will be 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 this option is omitted, then only the intercept term is estimated. The “Parameter Estimates” table in the displayed output gives 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".
The following options can be specified in the ZEROMODEL statement following a slash (/):
specifies the distribution function used to compute probability of zeros. The supported distribution functions are as follows:
specifies logistic distribution.
specifies standard normal distribution.
If this option is omitted, then the default ZI link function is logistic.
specifies a variable in the input data set to be used as a zero-inflated (ZI) offset variable. The ZI offset variable is included as a term, with coefficient restricted to 1, in the equation that determines the probability () of a zero count. The ZI offset variable cannot be the response variable, the offset variable (if any), or one of the explanatory variables. The name of the data set variable used as the ZI offset variable is displayed in the “Model Fit Summary” output, where it is labeled as "Inf_offset".
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