Hey @replyall/ @PJVogt! Apropos this week's ep, I have a harrowing tale.

Last year, our daughter (then 11) was at the barn near her school where she volunteers with the horses and gets lessons. It was getting close to dinner so I texted to see when she wanted to be picked up.

After a while I realized she hadn't answered so I tried to call her and got voicemail. I called the barn and the office was closed for the night. I called her friends and they'd all gone home too.

So we hit Find My Phone, and it said she was 13 miles away, on a street near a freeway entrance. My wife got in the car and started fighting traffic to get there while I called the sheriff - the phone was outside city limits - and frantically texted everyone.

Meanwhile, my wife's in tears driving, ringing our daughter's phone over and over and I'm hitting reload on Find My Phone and not getting any additional detail. We are freaking the fuck out.

Finally, in the midst of this - just as the cops are pulling up to the barn! - our daughter calls to say she's at the barn and what's going on? Her phone was in her bag.

Here's what we think happened. Your phone gets your location lots of ways - nearby wifi networks, GPS, and IP address assigned by your carrier. The LTE coverage near the barn is terrible and her phone was not getting reliable data.

When we hit Find My Phone, the server tried to query the phone for an accurate location fix, but it wasn't reachable. So Find My Phone checked the last IP address that had been assigned to the phone by the LTE carrier and did a lookup on that.

That carrier's IP range was not well fixed - they distribute IPs across a wide region without regard to narrow subzones. So the IP lookup response is, "The device is somewhere in this gigantic region."

(This was probably exacerbated by the fact that we use Ting and Fi, both MNVOs, who hop around lots of different carriers' networks)

But Find My Phone's UI has two responses: "The device is at this precise location" and "I don't know where this device is AT ALL." It has no way to signal, "The device is somewhere in this general range."

Nor is there a way to say, "This device's location is being estimated by IP address" vs "This device was reached in realtime and did a more accurate GPS or wifi-based fix."

The way Find My Phone resolves a location from a region is to drop a pin at the center of that region: in our case, a freeway onramp that made it look like our 11 year old was locked in a kidnapper's trunk and speeding out of town.

We KNOW the barn's coverage is spotty. If Find My Phone had told us, "I can't locate this device because its internet is offline," we'd have just driven over to the barn and gotten the kid. It's 5 minutes away from us!

But because it gave us a WRONG answer, based on an estimate, and didn't make it clear that it WAS an estimate, we had one of the worst hours of our lives.

There's a parallel here to machine-learning decision support: when it works well, you come to trust it. If it doesn't distinguish between high confidence and low confidence predictions, you will trust it when you shouldn't.

And the problem is WORSE when the system is more reliable: if Find My Phone works really accurately 99% of the time, then you're apt to attribute unexpected behavior to a scary situation, not a malfunction.

AKA: this is why I follow turn-by-turn directions even when it seems like they're advising me to drive off a bridge. I assume that since they haven't steered me wrong before, that I'm just assessing the situation incorrectly now.

Thank you for attending my TED Talk.