Model quality
Predictive Control : Process Modeling

A model is not acceptable if it does not fulfill its primary function, which is to "behave like" the object process it is supposed to represent. A second-order process with an oscillatory mode can never be correctly approximated by a first-order system, whatever the gain and time constant chosen. The model is too simple, its characterization too elementary. This results in an insufficient number of parameters defining the structure of the model.

The state distance is too large for the intended purpose. The model is then said to be under-characterized.