COUNTREG Procedure

DISPMODEL Statement

  • DISPMODEL dependent-variable tilde <dispersion-related-regressors></ option>;

The DISPMODEL statement specifies the dispersion-related-regressors that are used to model dispersion. This statement is ignored unless you specify DIST=COMPOISSON in the MODEL statement. The dependent-variable in the DISPMODEL statement must be the same as the dependent-variable in the MODEL statement.

The dependent-variable that appears in the DISPMODEL statement is directly used to model dispersion. Each of the q variables to the right of the tilde (~) has a parameter to be estimated in the regression. For example, let bold g prime Subscript i be the ith observation’s 1 times left-parenthesis q plus 1 right-parenthesis vector of values of the q dispersion explanatory variables (q 0 is set to 1 for the intercept term). Then the dispersion is a function of bold g prime Subscript i Baseline bold-italic delta, where bold-italic delta is the left-parenthesis q plus 1 right-parenthesis times 1 vector of parameters to be estimated, the dispersion model intercept is delta 0, and the coefficients for the q dispersion covariates are delta 1 comma ellipsis comma delta Subscript q Baseline. If you specify DIST=COMPOISSON in the MODEL statement but do not include a DISPMODEL statement, then only the intercept term delta 0 is estimated. The "Parameter Estimates" table in the displayed output shows the estimates for the dispersion intercept and dispersion explanatory variables; they are labeled with the prefix "Dsp_". For example, the dispersion intercept is labeled "Dsp_Intercept". If you specify Age (a variable in your data set) as a dispersion explanatory variable, then the "Parameter Estimates" table labels the corresponding parameter estimate "Dsp_Age". The following statements fit a Conway-Maxwell-Poisson model by using the regressors SEX, ILLNESS, and INCOME and by using AGE as a dispersion-related regressor: 

proc countreg data=docvisit;
   model doctorvisits=sex illness income / dist=compoisson;
   dispmodel doctorvisits ~ age;
run;

You can specify the following option after a slash (/):

SELECT=INFO=<(selection-options)>
SELECTVAR=INFO=<(selection-options)>

requests that the variable selection method be based on an information criterion. For a list of selection-options, see the section Options for Variable Selection Based on an Information Criterion. For more information about this type of variable selection, see the section Variable Selection Using an Information Criterion.

Last updated: June 19, 2025