Engine Monitoring

A method of monitoring a device, the method comprising: measuring one or more device operating parameters and a device performance parameter during one or more device operational periods; recording a plurality of data points, each data point comprising the device operating parameters and the associated device performance parameter; identifying one or more steady state regions in the device performance parameter and selecting a plurality of the data points from the one or more steady state regions in the device performance parameter; selecting one or more groups (40) of the steady state data points in the device operating parameter space by virtue of a predetermined density of the steady state data points in the device operating parameter space; and comparing the device performance parameter within the groups so as to identify a change in the performance of the device.