A respiratory mask assembly includes a respiratory mask and a forehead support secured to the mask via a joining member. The disk is controlled according to the result of determination such that it is rotated at a constant angular velocity or at a constant linear velocity and recording is executed. The outer perimeter of a seal has spacers interspersed with gaps. A running time window is defined, and within each window the time shifts are best fit to a straight-line function of time, one angle stack at a time. The body-worn device is assembled with the shell. Furthermore, the number of stocks in the stock index need not be a fixed value, but may depend upon how many stocks meet predefined criteria at any given point in time.