You can define a link function other than a built-in link function by using the FWDLINK statement. If you use the MODEL statement option LINK= to specify a link function, you do not need to use the FWDLINK statement. The variable identifies the link function to the procedure. The expression can be any arithmetic expression supported by the DATA step language, and it is used to define the functional dependence on the mean.
Alternatively, the link function can be defined by using programming statements (see the section Programming Statements) and assigned to a variable, which is then listed as the expression. The second form is convenient for using complex statements such as IF-THEN/ELSE clauses. The GENMOD procedure automatically computes derivatives of the link function required for iterative fitting. You must specify the inverse of the link function in the INVLINK statement when you specify the FWDLINK statement to define the link function. You use the automatic variable _MEAN_ to represent the mean in the preceding expression.