ABSTRACT
In this appendix, it will be shown that seeing models as measuring instruments provides a heuristic that can help improve a model that does not produce satisfactory test results. The heuristic consists of three interconnected aspects: comprehensive mathematical representation, invariance and calibration. This means that to assess a model one first tries to find out to what extent it is able to cover all the data. This is because to qualify as a measuring instrument the model must represent the whole data range. The model defines a mathematical system that fulfills the role of a scale, which must apply to the whole range. Second, of all possible comprehensive representations the model should represent the most invariant correlation under various circumstances. Third, calibration can ensure that the invariant relationship is gauged for appropriate invariants.
