1. A method for determining the validity of a respiratory effort waveform from a signal having non-respiratory artifacts comprising the steps of: (a) monitoring a respiratory effort waveform of the patient for at least one time parameter of the waveform characteristic of a non-respiratory artifact; and (b) comparing the parameter value with a predetermined limit having as a lower limit a fixed time period and as an upper limit a time period calculated from a baseline average of the parameter, the valid respiratory waveform indicated by the parameter value being between the upper limit and lower limit.