1. 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 comprising: receiving a plurality of data values captured over a period of time by each of said sensors; determining by a processing unit 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; extracting by a high pass filter a noise component of said correlation values; and outputting said calibration value determined based on the difference between said noise component and a reference noise value.