Each series contains 1800 evenly-spaced measurements of instantaneous heart rate from a single subject. 
The two subjects were engaged in comparable activities for the duration of each series. 
The measurements (in units of beats per minute) occur at 0.5 second intervals, so that the length of each series is exactly 15 minutes. 

Reference: http://reylab.bidmc.harvard.edu/DynaDx/case-study/hrv/ 

