We added calculation of distribution of differences in consecutive R-R intervals - so now we have a measure of how exactly these intervals change from beat to beat. There are many papers that use percentage of intervals that change by more than 50 millisecond for indicating stress level. But when we got full distribution, it was worth looking at!
We built a histogram of changes: first bar indicates how many R-R intervals vary by less then 10 millisecond, second bar - from 10 to 20 ms, third bar from 20 to 30 and so on. And I've went through normal, stressed and relaxed states. Here is the result:
So clearly this histogram is useful for indicating current state - but we need more experiments to determine what exactly it means. For now we just added it in the app interface, in two time scales, and will check how it behaves in different situations.
...also take a look how clear the signal is with the new version, even on the phone with lower data rate and some lost packets! ))