Traditionally, rotor blade vibration response measurements have been made using strain gauges. Rolls-Royce has a long experience of using strain gauge measurements to certify blades. In recent years, however, much effort has been invested in developing blade tip timing (BTT). The Rolls-Royce BTT system uses casing-mounted laser spot probes to measure the arrival times of the blade set. The vibration amplitudes of the blades are then inferred from these arrival times. BTT offers operational advantages over strain gauges and can measure every blade in the set. Previous back-to-back comparisons of strain gauges and tip timing have shown that both measurement systems measure the same trends in the data. However, initial analysis on a compressor rig showed that the two systems measured different response levels on the blades. This paper shows that both systems can be used with the finite-element model (FEM) to calculate the same amplitude-frequency level on the blade. This is provided that certain assumptions about the behaviour of the blades are true and that the uncertainties within both measurement systems are considered. The paper also presents a novel way of reporting the data to give a visual display of the quality of the data.