Models, again.............................

 You cannot derive a probability or a forecast or a policy recommendation from a model; the probability is meaningful, the forecast accurate or the policy recommendation well-founded only within the context of the model. . . . And our own experience in economics is that the most common explanation of a surprising result is the someone has made an error.  If finance, economics and business, models will never describe 'the world as it really is'.  Informed judgment will always be required in understanding and interpreting the output of a model and in using it in any large-world situation.

-John Kay and Mervyn King, Radical Uncertainty:  Decision=Making Beyond The Numbers