A method for determining a deviation value of a health parameter, in particular a capacity or efficiency parameter, of at least one component of a gas turbine. A pressure value and temperature value at a measuring point of the component is measured. A corrected pressure value is determined based on a predetermined pressure correction factor of the component. A corrected temperature value is determined based on a predetermined temperature correction factor of the component. An estimated actual value of the health parameter of the component is determined based on the corrected pressure value, the corrected temperature value and measured shaft speed. A nominal value of the health parameter of the component which is predicted based on at least one predetermined input parameter is determined. Finally, a deviation value of a health parameter is determined by comparing the estimated actual value and the nominal value.