The COUNTREG Procedure
- Overview
- Getting Started
-
Syntax
-
Details
Specification of Regressors Missing Values Poisson Regression Negative Binomial Regression Zero-Inflated Count Regression Overview Zero-Inflated Poisson Regression Zero-Inflated Negative Binomial Regression Computational Resources Nonlinear Optimization Options Covariance Matrix Types Displayed Output OUTPUT OUT= Data Set OUTEST= Data Set ODS Table Names
-
Examples
- References
| ZEROMODEL Statement |
zero-inflated regressors / optionsThe 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 is a function of 
, where 
is the 
vector of parameters to be estimated. (The ZI intercept is 
; the coefficients for the ZI 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 (/):
- LINK=value
-
specifies the distribution function used to compute probability of zeros. The following distribution functions are supported:
- LOGISTIC
specifies the logistic distribution.
- NORMAL
specifies the standard normal distribution.
If this option is omitted, then the default ZI link function is logistic.
- OFFSET=variable
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".