An apparatus for processing a set of data values, a data value representing a physiological measure of a body fluid at a time instant, comprising: an estimated probability function calculator for calculating an estimated probability function associated with the set of data values; a transform calculator for calculating a non-linear transform rule using a predetermined target probability function being different from the estimated probability function, so that the probability function of a set of transform data values is closer to the target probability function than the estimated probability function; and a transformer for applying the transform rule to the set of data values or to at least one further data value not included in the set of data values and sampled at the different time instant from the time instants for the set of data values to obtain at least one transformed value representing the physiological measure.