According to an aspect, there is provided a method of operating a non-invasive blood pressure, NIBP, monitor to measure the blood pressure of a subject, the NIBP monitor comprising a cuff, a pressure sensor for measuring the pressure in the cuff and for outputting a pressure signal representing the pressure in the cuff and a physiological parameter sensor, the method comprising obtaining a first measurement of a physiological parameter for the subject during inflation of the cuff, the first measurement being obtained from the pressure signal； obtaining a second measurement of the physiological parameter for the subject during inflation of the cuff, the second measurement being obtained from the physiological parameter sensor； comparing the first measurement and the second measurement； and estimating the reliability of a blood pressure measurement obtained by the NIBP monitor during inflation of the cuff based on the result of the step of comparing.
