From the study linked [https://link.springer.com/article/10.1007/s40279-024-02066-5] (with acronym meanings added inline for readability):
"The lowest pulse arrival time standard deviation (PAT σ) at 2.0 times the standard deviation corresponded to 88.4% of the Region of Practical Equivalence (ROPE) for SDNN (Standard Deviation of Normal-to-Normal intervals), and 21.4% for RMSSD (Root Mean Square of Successive Differences). As the standard deviation of PAT increases, the equivalence between photoplethysmography-derived "heart rate variability" (PRV) and electrocardiography-derived heart rate variability (HRV) decreases for both SDNN and RMSSD. The width of the highest density interval (HDI), which encompasses 95% of the posterior distribution, increases with increasing PAT σ. This increase occurs at a higher rate for RMSSD than for SDNN."
So for "how accurate?", grossly and irresponsibly oversimplifying, the Apple method is roughly 90% accurate, and devices using the inferior method are roughly 20% accurate.
I would be really interested to know of any devices not locked down to Apple ecosystem that also use this approach, if anyone has any insight.
So for "how accurate?", grossly and irresponsibly oversimplifying, the Apple method is roughly 90% accurate, and devices using the inferior method are roughly 20% accurate.
I would be really interested to know of any devices not locked down to Apple ecosystem that also use this approach, if anyone has any insight.