For this example, a three-layer tree structure was allowed, including [𝑃𝑁𝑃𝐴] as the only predictor variable. Due to the relatively low values of the measured reaction rates (10 -7 -10 -9 mol•L -1 •s -1 ), they were expressed as mmol•L -1 •h -1 . As shown, both errors are of 2 or 3 orders of magnitude higher when 𝐶 = 3, whereas similar training errors were obtained when 𝐶 = 5 and 𝐶 = 7. However, the lowest extrapolation error suggests that the model with complexity 𝐶 = 5 is the most suitable one for the description of the kinetic behaviour of the system.