The invention concerns a method for determining a calibration value indicating the extent of loss of calibration of a group of three or more sensors in a sensor network, the method involving receiving a plurality of data values captured over a period of time by each of the sensors, determining by a processing unit (404) at least one correlation value associated with each sensor, each correlation value corresponding to the correlation between the data values captured by the associated sensor and the data values captured by at least one other sensor; (410) a noise component of the correlation values and outputting the calibration value determined based on the difference between the noise component and a reference noise value.