A gas analysis apparatus, and a method for calibrating it and for compensating measurement errors, are disclosed. This method and apparatus are particularly suited for use during a cardiopulmonary exercise test by a test subject. The oxygen and carbon dioxide concentrations of the subject's breath are measured, and errors are compensated based on the results of previous calibration. These compensated measurements, as well as other physiological data monitored during the cardiopulmonary exercise test quantities calculated from these measurements, are presented as a series of graphs in a logical order to enhance their diagnostic and prognostic value. A facemask and headstraps are adapted for use with the gas analysis apparatus. The facemask possesses a plurality of pins fitting into corresponding holes in the headstraps, and the headstraps possess quick-release attachment means to provide for quickly securing the face mask to or removing it from a subject.