1. A method of monitoring a patient comprising: obtaining data indicative of the timing and position of at least part of a patient during at least one breathing cycle; using the obtained data to generate a model of the variation in position of the at least part of a patient during the at least one breathing cycle, the model being responsive to receipt of input data indicative of position and timing to determine a set of weights for modelling a representation of the input data; obtaining further data indicative of the timing and position of the at least part of a patient during a subsequent period of breathing; determining a set of weights for modelling a representation of the further data utilising the generated model; calculating a plausibility measure indicative of the probability of the further obtained data arising based on the set of weights determined for modelling the representation of the obtained data; and comparing the plausibility measure of with a threshold value.