|The GENMOD Procedure|
You can specify a probability distribution other than those available in PROC GENMOD by using the DEVIANCE and VARIANCE statements. You do not need to specify the DEVIANCE or VARIANCE statement if you use the DIST= MODEL statement option to specify a probability distribution. The variable identifies the deviance contribution from a single observation to the procedure, and it must be a valid SAS variable name that does not appear in the input data set. The expression can be any arithmetic expression supported by the DATA step language, and it is used to define the functional dependence of the deviance on the mean and the response. You use the automatic variables _MEAN_ and _RESP_ to represent the mean and response in the expression.
Alternatively, the deviance function can be defined using programming statements (see the section Programming Statements) and assigned to a variable, which is then listed as the expression. This form is convenient for using complex statements such as IF-THEN/ELSE clauses.
The DEVIANCE statement is ignored unless the VARIANCE statement is also specified.