MODEL
dependent = <regressors> </ options> ;
The MODEL statement specifies the dependent variable and independent covariates (regressors) for the regression model. If
you specify no regressors, PROC COUNTREG fits a model that contains only an intercept. The dependent count variable should
take on only nonnegative integer values in the input data set. PROC COUNTREG rounds any positive noninteger count values to
the nearest integer. PROC COUNTREG ignores any observations with a negative count.
Only one MODEL statement can be specified. The following options can be used in the MODEL statement after a slash (/).

DIST=value

specifies a type of model to be analyzed. If you specify this option in both the MODEL statement and the PROC COUNTREG statement,
then only the value in the MODEL statement is used. The following model types are supported:
 POISSON  P

Poisson regression model
 NEGBIN(P=1)

negative binomial regression model with a linear variance function
 NEGBIN(P=2)  NEGBIN

negative binomial regression model with a quadratic variance function
 ZIPOISSON  ZIP

zeroinflated Poisson regression. The ZEROMODEL statement must be specified when this model type is specified.
 ZINEGBIN  ZINB

zeroinflated negative binomial regression. The ZEROMODEL statement must be specified when this model type is specified.

NOINT

suppresses the intercept parameter.

OFFSET=variable

specifies a variable in the input data set to be used as an offset variable. The offset variable appears as a covariate in
the model with its parameter restricted to 1. The offset variable cannot be the response variable, the zeroinflation offset
variable (if any), or one of the explanatory variables. The Model Fit Summary gives the name of the data set variable used
as the offset variable; it is labeled as “Offset.”
Variable Selection Options

SELECTVAR=(option)

specifies variable selection based on an information criterion. For more information, see the section Variable Selection. You can specify the following options:

DIRECTION=FORWARD  BACKWARD

specifies the searching algorithm used in the variable selection method. The default is FORWARD.

CRITER=AIC  SBC

specifies the information criterion used for the variable selection. The default is AIC.

MAXSTEPS=value

specifies the maximum number of steps allowed in the search algorithm. The default is infinite; that is, the algorithm does
not stop until the stopping criterion is satisfied.

LSTOP=value

specifies the stopping criterion. The value represents the percentage of decrease or increase in the AIC or SBC that is required for the algorithm to proceed; it must
be a positive number less than 1. The default is zero.

RETAIN(value)

specifies a list of regressors to be retained in any model that the variable selection process considers.
The following rules apply to the way in which regressors are handled when you specify more than one MODEL statement and use
the SELECTVAR option.
If you do not specify the SELECTVAR option in a MODEL statement, then all regressors in the original model are included in
any model that the variable selection algorithm considers. In other words, omitting the SELECTVAR option is equivalent to
specifying the option SELECTVAR=(RETAIN(allregressors)).
If you specify the SELECTVAR option without any =(option) clause in a MODEL statement, then all regressors in that model (other than the Intercept, if present) are eligible for potential
exclusion as the variable selection process is executed.
The following example shows how to use the SELECTVAR statement. There are ten possible regressor candidates. Out of the ten
candidates, five are selected using the AIC criterion.
proc countreg data=one;
model y = x1x10 /selectvar=(direction=forward criter=AIC maxsteps=5);
run;

CORRB

prints the correlation matrix of the parameter estimates. The CORRB option can also be specified in the PROC COUNTREG statement.

COVB

prints the covariance matrix of the parameter estimates. The COVB can also be specified in the PROC COUNTREG statement.

ITPRINT

prints the objective function and parameter estimates at each iteration. The objective function is the negative loglikelihood
function. The ITPRINT option can also be specified in the PROC COUNTREG statement.

PRINTALL

requests all printing options. The PRINTALL option can also be specified in the PROC COUNTREG statement.
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