Surveillance, Environmental
Eric Järpe
Encyclopedia of Environmetrics, John Wiley & Sons, 2002

The first lines
In many environmental random processes it is relevant to consider a sudden shift in a parameter of the probability distribution of the random variables. This parameter may be a mean level of the process or the natural variation of the variables. Suddenly the parameter can shift to some other, commonly higher, value or to another, commonly increasing, function in time and/or space. To prevent or mitigate the fatal consequences such shifts could imply, it is often of utter importance to detect the change as accurately and quickly as possible. Therefore, stopping rules which are functions of the random process data observed are evaluated, on-line or retrospectively, in order to indicate a systematic change. Different shift processes are mentioned. An example with data from the Swedish Radiation Safety Authority is given.