Abstract: A critical parameter for a device is generated as follows. The critical parameter is modeled. When the device is operated under steady-state conditions, the modeled parameter is corrected by a measured parameter known to be accurate under steady state operating conditions. The correction accounts for errors which may be present in the synthesis of the critical parameter and normal anticipated performance variation of the device. When the device is operated under transient conditions, however, the correction of the modeled parameter is based upon previous steady-state conditions. Thus, the apparatus generates a theoretical parameter during transient operating conditions.