Measuring metrics is easy, it's the algorithm on the backend that matters.

There's a reason why Oura rings are expensive and it's not the hardware - you can get similar stuff for 50€ on Aliexpress.

But none of them predicted my Covid infection days in advance. Oura did.

A device like the Apple Watch that's on you 24/7 is good with TRENDS, not absolute measurements. It can tell you if your heart rate, blood oxygen or something else is more or less than before, statistically. For absolute measurements it's OK, but not exact.

And from that we can make educated guesses on whether a visit to a doctor is necessary.

I'm curious how the ring detected it in advance? I also discovered my Covid when I looked at my Garmin watch and my resting heart rate was 100, until then I had thought I had too much sun that day.