There is provided a method for use in monitoring a physiological characteristic of a subject, the method comprising receiving a value of the physiological characteristic of the subject; determining whether the received physiological characteristic value is normal using a range of normal variability of the physiological characteristic for the subject calculated using a set of previous values of the physiological characteristic for the subject; and in the event that the received physiological characteristic value is determined to be normal, updating the range of normal variability using the received physiological characteristic value.