Abstract: A decision making process for evaluating performance shifts identified during a diagnostic trend analysis of gas turbine engine data includes first determining whether each performance shift is actionable, and then determining what maintenance action is required for actionable performance shifts. The maintenance action is determined by identifying potential causes of the actionable performance shifts and ranking the identified potential causes in most likely order. A manual is provided for guiding a user through the decision making process. The manual includes a flow chart flow chart setting forth the steps of the decision making process and a number of tables used in the process.