A method of monitoring an operating parameter of an apparatus. The method comprises taking a measurement of the operating parameter of the apparatus on multiple occasions. The mean and standard deviation of a set of threshold determining values is then determined, the threshold determining values each being dependent on at least one of the operating parameter measurements. Thereafter an event threshold, dependent on the mean and standard deviation of the set of threshold determining values is determined. The operating condition value is monitored, where the operating condition value at any given time is dependent on at least one of the operating parameter measurements. Finally an event signal is generated if the operating condition value is in a regime beyond the event threshold.