Apple Watch heart rate variability inconsistencies

Harvard biostatistics researcher JP Onnela has discovered that it is not possible to use Apple Watches for research into heart rate variability after finding the data collected was inconsistent. 

According to Onnela, updates to Apple Watch algorithms mean 'the data from the time period can change without warning'.  'These algorithms are what we would call black boxes — they’re not transparent. So it’s impossible to know what’s in them' he argues.