A method of monitoring a system such as a machine, industrial system, or human or animal patient, to classify the system as normal or abnormal, in which a time-series of measurements of the system are regarded as a function to be compared to a model of normality for such functions. The model of normality can be constructed as a Gaussian Process and test functions compared to the model to derive the probability that they are drawn from the model of normality. A probability distribution for the expected extrema of sets of functions drawn from the model can also be derived and the probability of any extremum of a plurality of test functions being an extremum of a set derived from the model of normality can be obtained. The system can be classified as normal or abnormal based on the extreme probability distribution. Test functions with fewer data points can be compared to the model of normality by marginalisation with respect to the missing data points.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method of monitoring a system to classify states of the system as normal or abnormal, comprising the steps of: obtaining a time series of measurements of the system state, defining as a test function the time series of measurements, obtaining a model which defines a probability distribution over functions each function consisting of time series of measurements of a system in a normal state; comparing the test function to the model to obtain a probability that the test function corresponds to the model; and classifying the system as abnormal if the obtained probability for the test function is lower than a predetermined threshold. 2. A method according to claim 1 wherein the model is a Gaussian Process. 3. A method according to claim 1 wherein the probability distribution over functions is a multivariate Gaussian with number of variables equal to number of measurements in the time series. 4. A method according to claim 1 wherein when the test function has missing measurements compared to the time series defining the model, the probability distribution is marginalised with respect to missing measurements. 5. A method according to claim 1 wherein an extreme function distribution is defined with respect to the model and said step of comparing the test function to the model to obtain a probability comprises comparing the most extreme of a plurality of test functions to the extreme function distribution to obtain an extremum probability that the most extreme function appears as an extremum of a corresponding plurality of functions drawn from the model. 6. A method according to claim 5 wherein said extremum probability is compared to threshold and if it is lower than the threshold the system is classified as abnormal. 7. A method according to claim 1 wherein the system is human or animal and the measurements are vital signs measurements. 8. A method according to claim 1 wherein the system is a machine or industrial plant. 9. A system monitor comprising an input for receiving a time series of measurements of the system state, a processor adapted to execute the method of claim 1, and an output for outputting the classification result. 10. A computer program comprising program code means for executing on a programmed computer system the method of claim 1.