So, according to this, the false positive rate for the Apple Watch in detecting atrial fibrillation is 0.04% (99.6% correct)

This means that, on average, Apple Watches will be wrong more than 80% of the time

Sound counterintuitive? This is the issue with population screening
It's actually fairly basic maths

The population prevalence for atrial fibrillation is about 1%
Assuming that the false negative rate is 0% (this is pretty close, it was actually 2%), in a sample of 1000 people you'll get 10 with AF who will receive the correct diagnosis
Now, the false negative rate is 0.04%

So, in the remaining 990 people who are tested, 40 will receive positive results
Keeping track, that means 4 out of every 5 people who are diagnosed using this app will be diagnosed incorrectly
And remember, these people will then likely go on to receive unnecessary tests, interventions, and potentially medications

Some of them will probably die
This is why population screening is a fine art

Even if your test is FANTASTIC, you have to be really damn careful

Because otherwise you can KILL PEOPLE
P.S. just in case this wasn't elucidated well above - 1% prevalence means that 10 in 1,000 people have the disease and AF=atrial fibrillation
P.P.S - As has been pointed out, I got my simple maths incorrect here

Serves me right for being overconfident!

The true negative identification should be 4, thus the watch is only wrong ~1/3 of the time

Still a fascinating phenomenon
This false identification rate can be reduced if the proportion of the population at risk increases

For example, if 10% of the population have AF, then the watch only gets it wrong a tiny percentage of the time
This is why screening is usually only recommended in very high-risk populations
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