New York City is approaching a 3% COVID positivity rate – but how is that number calculated?

New York City and New York State pull test data from the same source, but treat and analyze it slightly differently. AFP via Getty Images

New York City Mayor Bill de Blasio said on Friday parents should prepare for the possibility that schools will be closed on Monday if COVID-19 infections exceed a critical measure over the weekend – something he said warned that the city was “close enough”.

But at what point? The city’s Ministry of Health and Mental Hygiene reported on Friday that the seven-day moving average positivity rate had climbed to 2.83% – in fact, just below the critical 3% threshold that would trigger the closure of schools. The state health ministry, however, reported the same metric for the five boroughs at 2.3%, significantly lower when even the slightest increase could be decisive.

Any data nerd (ahem, like us) might be confused as to how, when so much is at stake, the city and state figure could diverge by more than half a percentage point.

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The answer is more than a little technical, but it all depends on where the data comes from and how it is processed.

The city and state pull infection data from a state repository known as the Electronic Clinical Laboratory Reporting System, or ECLRS (pronounced like French pastries), said Corinne Thompson, an epidemiologist at the ministry. of Health and mental hygiene of the city.

“If you’re a New York state resident, you’ll go, you’ll take a test, the swab will be sent to a lab,” she said. “Then the lab is legally obligated to report to that system, which is ECLRS.”

Once a day, the state sends the city the test data it received for patients in the five boroughs, according to Thompson and a spokesperson for the state health department.

The next step is largely where the deviation occurs. New York State ranks a case based on when the test result arrived, while New York City ranks each case based on when a person was tested in the doctor’s office. Same raw data, slightly different analyzes.

“The reason we’re doing this is because we think it’s the most epidemiologically relevant, because it’s the closest to when someone really started to feel sick,” Thompson said.

New York City is also doing a bit more data collection which could result in a slight difference between city and state numbers for the five boroughs. For example, the city sometimes meets a patient who actually lives in New Jersey but has been tested in New York. This fix is ​​not being reported to the state, she said.

There are also hiccups like twins – same birthday, same household, often similar names – that can trigger algorithmic data processing and require manual cleaning. But that tune-up is largely insignificant when it comes to final metrics.

So which number is the most important?

When it comes to school closures, unfortunately for anxious parents of public schools, it is the sliding seven-day positivity rate as reported by the city at 2.83%.

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If the city’s dedicated data collectors had totaled that figure at 3% or more this weekend, all public schools would be closed on Monday, excluding community pre and 3-K providers, the mayor said on Friday. to radio show host Brian Lehrer. .

“It’s not something that any parent, you know, wants to have to do, but we have to prepare and parents should have a plan for the rest of November,” he said. .

Gov. Andrew Cuomo, who has overturned de Blasio on school decisions in the past, claimed in his own public briefing that the mayor would be in his power to shut down schools – apparently even if the state’s metric remains below 3%.

“This is the deal that New York announced,” Cuomo said Friday. “I hope the mayor, teachers and parents will work together to reopen the schools as quickly as possible.