Descriptive models describe things as they are or as they are or as they are believed to be.
These models are typically mathematically based. Descriptive models are extremely useful in DSS for investigating the consequences of various alternative courses of action under different configuration of inputs and processes.
However , because a descriptive analysis checks the performance of the system for a given set of alternatives (rather then for all alternatives), there is no guarantee that an alternative selected with the aid of descriptive analysis is optimal.
In many cases , it is only satisfactory.