Observability and Structural Identifiability of Nonlinear Biological Systems

Figure 1

Illustration of observability and structural identifiability issues. Top: diagram and equations of the ‘IG model’ of the glucose-insulin system [35]. If glucose concentration (G) and -cell mass are measured, the parameters and are structurally unidentifiable: the bottom plots show that different combinations of and values yield identical curves of G and , so it is not possible to distinguish between them as long as the product , which is structurally identifiable, remains constant. Likewise, in this case insulin concentration (I) is an unobservable state: it is not possible to determine which of the two time courses of I shown in the lower left plot is the true one.