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empirical model is a description that is driven by the data we have, whereas
a theoretical model is derived from theoretical understanding of the system
under study. For example, the perfusion limited physiological model studied
in Section 4.4.2 can be considered to be a theoretical model since it starts
from our knowledge of the human body, whereas the modelling of a drug dis-
cussed in Chapter 5 to a large extent is an empirical modelling approach. A
theoretical model may be very complex, and a simplification of the model may
be needed. How this is done depends on the purpose of the modelling, but
an illustration is the approximation of the theoretical physiological model in
Section 4.4.2 by a mamillary three-compartment model that ...