1. A method of selecting, for storage in a memory, data representing nonlinear changes in a monitored parameter, comprising: monitoring a parameter to obtain a signal representing a measurement of the parameter; converting the signal into a measurement response, Z, representing the deviation of the measurement of the parameter from a hypothetical straight line; determining a linear region within an allowable variation from the hypothetical straight line, including: selecting a relative cost, C, between a false alarm event, wherein a measurement response resulting from inherent randomness would be saved, and a miss event, wherein a measurement response resulting from a true nonlinear change in the monitored parameter would not be saved; defining a first probability, P.sub.H0, that a measurement response is the result of inherent randomness; defining a second probability, P.sub.H1, that a measurement response is the result of a true change in the monitored parameter; and defining from the relative cost, the first probability and the second probability an offset value, X, as the allowable variation by which the linear region is determined; comparing the measurement response Z and the offset value X to determine when Z is greater than X and thus to determine when Z is outside the linear region so that Z then represents a nonlinear change in the monitored parameter; and storing Z in the memory when Z is greater than X.