Yes, you're right. It's usually the standard deviation, but there are dozens of competing, correlated ways to calculate it (rmsd etc). But the underlying data i

Yes, you're right. It's usually the standard deviation, but there are dozens of competing, correlated ways to calculate it (rmsd etc). But the underlying data is the volatility of beat-to-beat intervals. About half of my class at university (me included) actually took a deep breath when they learned about HRV because we had all at some point worried about our heart rate seeming to erratically change from second to second. We all just never worried enough to ask anybody.