Public health does not understand math

Proximity technology is controversial, particularly among some Americans who are unwilling to share personal data for privacy reasons and skeptical of the big tech companies offering the service. But it’s been embraced in some places, including Scotland, where a new app was reportedly downloaded 600,000 times.

Source: Your phone could determine if you’ve been exposed to coronavirus. Oregon’s ready to embrace the tech – oregonlive.com

600,000 sounds impressive based on intuition (everything in public health seems to be based on intuition).

The population of Scotland is estimated as 5.5 million.

600k of 5.5 million is 10.9% of the population.

If we assume everyone has a compatible smart phone (which is a false assumption), then the probability that an individual has the app is 10.9%.

The probability that two people have the app is 10.9% x 10.9% or 1%. It takes two to have a detection.

Thus, while 600,000 sounds like a lot of downloads, it will detect up to 1% of potential cases. Which is indistinguishable from ZERO.

Only in the mind of public health does this sound like “embraced”. But you know, “intuitively” this sounds like a great idea, right?

Oh My God. This news article says there is up to a FALSE POSITIVE RATE OF 45%!

The Turing team estimate that 7 out of 10 contacts will be correctly notified if they are within 2 meters and for longer than 15 mins. Detection rates within 5 metres achieve 99.3% accuracy of all contacts recorded, with a false positive rate of 45% and a true positive rate of 69%. 

Oh My God. Start thinking through the Bayesian matrix here. On My God.

Bluetooth-based smart phone contact tracing is an idea that seemed intriguing when it was proposed last spring. It was a way for the tech sector to join the virtue signaling crowd by doing something for the pandemic. The tech, however, is a poor solution. Most of the media confused this with network-side tracing technology (the same way they confused an N95 mask as equivalent to a DIY home made cloth mask when reviewing “mask studies”.) In many countries, like S. Korea, tracing is done using the network to locate all phones, including dumb phones and then follow up with in person contact tracing. But that method violates privacy rules in North America, the EU and other countries. So tech came up with the Bluetooth idea – and by damned, everyone is so invested they will absolutely drive it forth even though preliminary tests have found it to have little value.