Methods and apparatus are disclosed to automatically generate posynonomial performance parameter models for linear and non-linear systems such as electronic circuit characteristics, based on numerical simulations, measurements or observation. The resulting models can be used for automated optimisation of the systems, e.g. optimization-based sizing of electronic circuits so that their performance meets the set of specifications imposed by the designer. They can also be used for optimizing the performance of the system such as the performance of an electronic circuit or to adjust or control the operation of the circuit. The methods and apparatus can be applied to any physical (e.g., electrical, chemical, mechanical, biological) or non physical (e.g., economical, financial, banking) system with which the input is performance values of the system rather than an analytical definition of the system, e.g. the input can be performance data obtained by simulation and/or by measurement and/or observation.