Tuning coronavirus exposure warnings

 In The Washington Post, Geoffrey A. Fowler described a smartphone app that alerts a user when the user has been in close contact with someone who reports a positive COVID-19 test result.  The app uses Bluetooth technology to track which other smartphones one has been near (but not the physical locations of the contacts).

 Fowler's article also mentioned the problem of false positives, a typical problem in the design of any warning system.  The system is meant to alert a user if they have had a close contact with someone who tests positive.  To do that, the app must define a "close contact" based on the signal strength (stronger = closer) and the duration of the contact.  If these parameters are set too "low" so that weak signals and short encounters are considered "close contacts," then a user may get too many alerts (false positives); on the other hand, if the parameters are set too "high," then a user may not receive an alert about a risky encounter (a false negative).  

The article quoted Jenny Wanger of the Linux Foundation Public Health, who said, "We are working as a community to optimize it and to figure out how to get those settings to be in the right place so that we do balance the risk of false positives with the getting notifications out to people who are at risk."

Note that an alert here is not a positive test result for the user, only a warning that one was near someone with a positive test result and thus may have been exposed.  The costs of false positives and false negatives are subjective, of course.  At this point in the pandemic, a false positive, which may cause a user to quarantine or limit his activities unnecessarily, may be more costly that a false negative for someone who is taking precautions while doing typical activities and is likely having many brief, low-risk encounters.  This type of user may prefer to know only about really close contacts that have a higher risk of transmission.

The opposite may be true for someone who is significantly at risk for becoming seriously ill and has very few contacts in a typical day.  Then, a more sensitive (but less specific) system may be more appropriate.

Thus, it would be useful for users to have the ability to set the warning threshold based on their risk preferences, similar to the way financial advisors ask investors about their risk tolerance.

Too Many Warnings?

At 5:35 P.M. on September 17, 2018, the University of Maryland Police Department (UMPD) sent a tornado warning by email and text message to university students, faculty, and staff: "A Tornado Warning has been issued for the UMD campus. The sirens will be activated. Seek shelter immediately, avoid windows."  This message did not mention that the warning was issued by Accuweather.  The National Weather Service (NWS) did not issue a tornado warning.  At 6:04 P.M., the UMPD sent the following message: "The Tornado Warning that was issued by AccuWeather is now cancelled as of 6:00 PM."

Part of Accuweather's business is generating severe weather alerts for its clients. Its website touts the benefits of Accuweather's warnings and its "null tornado notifications" that a NWS tornado warning will not affect a client's facility.  The website discusses the benefits of knowing that a tornado warning is not a direct threat, so a shutdown is not necessary, and claims that Accuweather's false alarm rate is lower than the NWS false alarm rate (10% to 80%).

When one organization issues a warning while the other does not, the inconsistency could decrease trust in warnings, increasing the likelihood of ignoring warnings.
An article in The Washington Post about the false alarm (Angela Fritz and Sarah Larimer, "Red flags at U-Md. over false alarm for tornado," September 19, 2018) quoted Gary Szatkowki, a former NWS meteorologist, who asked "What do we want to do about a weather organization issuing a tornado warning when the Weather Service does not?"

Although the University of Maryland is a large institution (approximately 40,000 students), its cost of a false alarm is smaller than the same cost for a factory (where a shutdown directly impacts throughput and revenue).  Moreover, many students and others are outdoors, and the university has a strong desire to be viewed as a safe place, so the potential impact is large.

In that case, the university should be willing to tolerate more warnings.  Too many warnings?  No, in this case.

How to React to an Earthquake

One cannot prevent an earthquake, but one can protect people and property from its impact.  Traditionally, this means designing and building structures that will survive the earthquake without damage (preventive actions).  Now, because of earthquake warning systems, contingency planning is becoming feasible.

Earlier this month, The Washington Post's Eric Niiler wrote about technology that can tell that an earthquake is about to happen.  The November 3 article ("Last-minute warnings may make quakes less destructive") states that a combination of ground sensors and satellite-based instruments can give a warning a minute or two before the earthquake occurs.

The technology works by detecting the P-waves that precede the earthquake's shock (the sound waves are an earthquake precursor).  The ShakeAlert earthquake early warning system for the west coast of the United States was developed by the United States Geological Survey (USGS) and university partners.  According to Niiler, Chile, Mexico, China, Taiwan, Turkey, and Israel are developing similar systems.

Although a minute is not enough time to evacuate large buildings or a city, it is enough to shut down (or put into a safe mode) critical systems such as trains, gas lines, elevators, tunnels, and bridges.  Unfortunately, it is not enough time to send text messages to millions of people simultaneously, so earthquake alerts go only to emergency agencies, utilities, and similar high-priority organizations.

For more about ShakeAlert, see the fact sheet at http://pubs.usgs.gov/fs/2014/3083/pdf/fs2014-3083.pdf