1. A method for operating an industrial plant, comprising: having a rule-based expert system that comprises a set of rules for evaluating the operation of a process unit in the industrial plant; having a model-based expert system that comprises a statistical mathematical model for evaluating the operation of the process unit in the industrial plant; applying the model-based expert system to evaluate the operation of the process unit using statistical data on the operating conditions of the process unit; obtaining a statistical analysis result from the model-based expert system; applying the rule-based expert system to identify a possible abnormality in the operation of the process unit using: (a) data on the operating conditions of the process unit, and (b) the result from the model-based expert system; sending a first message from the rule-based expert system regarding an abnormality to an operator interface; suppressing a second message from the model-based expert system, wherein the suppressed message relates to the abnormality and is redundant to the first message; and adjusting the operation of the process unit based on the message relating to the abnormality.