The authors are concerned with the interpretation of the instantaneous signals from an X hot-wire probe used for velocity component measurements. A detailed calibration study was carried out to identify the correct velocity and yaw response of a typical plated X hot-wire probe. The calibration data identified an accurate calibration relationship for the X hot-wire probe, and enabled the development of a related look-up inversion method. This provided a reference for the assessment of the accuracy of common signal analysis methods. Using a step-by-step approach, the errors caused by the assumptions of constant values for the offset and exponent in a power law relationship were first identified. Then, the errors relating to the introduction of the concept of an effective velocity V_eff and analysis in terms of V_eff² were determined and finally the errors in the sum and difference method based on V_eff. The authors demonstrate that these errors cannot be neglected, even at low turbulence intensity, if accurate flow measurements are required.