A method for creating, maintaining and using an expert system segregates the rulebase for the expert system into a standard rulebase and a calibration rulebase. The standard rulebase is stored in a read-only file, while the calibration rulebase is stored in a read/write file. An identical standard rulebase is provided for several units of a model or pieces of equipment of one type, while each unit has its own calibration rulebase. The operator of a unit is given access to the calibration rulebase to make modifications to the calibration properties for that particular unit. The calibration rulebase for a specific unit is then merged with the standard rulebase to form the application rulebase used in evaluating the unit.