Understanding the structure of a large econometric model is rather like the art of winetasting or like the art of playing a musical instrument. The quality of a wine results from a complex combination of various elements such as its colour which should be clear and crystalline, its smell which can be decomposed into a general aroma and a variety of particular characteristics, more or less persistent depending on the type and the age of the wine, its taste, of course, which again is a complex system whose equilibrium and charm depend on the whole set of ingredients: alcohol, tannin, glycerine,...