relates a vector of [[independent variables]], ''':<math>\mathbf{x'''}</math>, and its associated observed [[dependent variables]], '''y'''. The function ''f'' is nonlinear in the components of the vector of parameters ''β'', but otherwise arbitrary. For example, the [[Michaelis–Menten]] model for enzyme kinetics has two parameters and one independent variable, related by ''f'' by:{{efn|This model can also be expressed in the conventional biological notation:
:<math> v = \frac{V_\max\ [\mbox{S}]}{K_m + [\mbox{S}]} </math>
