A method of measuring respiratory effort of a subject, the method comprising obtaining a thorax effort signal, the thorax effort signal being an indicator of a thoracic component of the respiratory effort, the thorax effort signal including a volume-contributing thoracic component (V ST) and a thoracic paradox component (P T), the thoracic paradox component (P T) representing a non-volume-contributing thoracic component of the respiratory effort, and the volume-contributing thoracic component (V ST) representing a volume-contributing component of the respiratory effort, and obtaining an abdomen effort signal, the abdomen effort signal being an indicator of an abdominal component of the respiratory effort, the abdomen effort signal including a volume-contributing abdominal component (V SA) and an abdomen paradox component (P A), the abdomen paradox component (P A) representing a non-volume-contributing abdominal component of the respiratory effort, and the volume-contributing abdominal component (V SA) representing a volume-contributing component of the respiratory effort, wherein the abdominal paradox component (P A) is negatively proportional to the thoracic paradox component (P T), the method further comprising the following steps performed by a processor: performing a calibration of the thorax effort signal and the abdomen effort signal, which allows for differing levels of obstructive breathing during the calibration, based on the obtained thorax effort signal and the abdomen effort signal by weighting the thorax effort signal by a first weight factor (k T) and weighting the abdomen effort signal by a second weight factor (k A) to obtain a volume-proportional signal (V Sw) which is the weighted sum of the thorax and abdomen effort signals, the volume-proportional signal (V Sw) being proportional to the actual respiratory volume of the respiratory effort, characterized by calculating a ratio between an amplitude or power, or a transformation function of the amplitude or power of said volume-proportional signal (V Sw) and a sum of the amplitudes, power, or a transformation function of the thorax effort signal weighted by the first weight factor (k T) and the abdomen effort signal weighted by the second weight factor (k A), and selecting the first and second weight factors (k T and k A) that minimize said calculated ratio. the thorax effort signal including a volume-contributing thoracic component (V ST) and a thoracic paradox component (P T), the thoracic paradox component (P T) representing a non-volume-contributing thoracic component of the respiratory effort, and the volume-contributing thoracic component (V ST) representing a volume-contributing component of the respiratory effort, and obtaining an abdomen effort signal, the abdomen effort signal being an indicator of an abdominal component of the respiratory effort, the abdomen effort signal including a volume-contributing abdominal component (V SA) and an abdomen paradox component (P A), the abdomen paradox component (P A) representing a non-volume-contributing abdominal component of the respiratory effort, and the volume-contributing abdominal component (V SA) representing a volume-contributing component of the respiratory effort, wherein the abdominal paradox component (P A) is negatively proportional to the thoracic paradox component (P T), the method further comprising the following steps performed by a processor: performing a calibration of the thorax effort signal and the abdomen effort signal, which allows for differing levels of obstructive breathing during the calibration, based on the obtained thorax effort signal and the abdomen effort signal by weighting the thorax effort signal by a first weight factor (k T) and weighting the abdomen effort signal by a second weight factor (k A) to obtain a volume-proportional signal (V Sw) which is the weighted sum of the thorax and abdomen effort signals, the volume-proportional signal (V Sw) being proportional to the actual respiratory volume of the respiratory effort, characterized by calculating a ratio between an amplitude or power, or a transformation function of the amplitude or power of said volume-proportional signal (V Sw) and a sum of the amplitudes, power, or a transformation function of the thorax effort signal weighted by the first weight factor (k T) and the abdomen effort signal weighted by the second weight factor (k A), and selecting the first and second weight factors (k T and k A) that minimize said calculated ratio.