Observability and Structural Identifiability of Nonlinear Biological Systems

Figure 2

Output of the two-compartment model of (19) for two different parameter vectors ( given by , and given by ), and two different inputs, and (where stands for time). With a constant input both parameter vectors are indistinguishable from the model output (there is actually an infinite number of pairs of indistinguishable parameter vectors), and the parameters are structurally unidentifiable. However, with a ramp input two different parameter vectors yield two different model outputs; in this case the parameters are structurally identifiable.