Is the positivity rate getting short thrift?

The Quarantine Zone
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I did a search but couldn’t find another thread on this, apologies if I’m rehashing.

IANAS, obviously, but I’m fairly good with math. Every morning I check the numbers for Ontario to see how many new cases have been detected the previous day. For two weeks now Ontario has been under 200 new cases a day. During our third wave earlier this year, we were in the thousands; at one point in April our 7-day average was 4,341 new cases. So obviously things are much better, as we enter the second week of the third stage (of the seventh son…) of reopening.

But it took me a long time to figure out that the positivity rate was just not getting as much attention. As we were climbing down from those awful heights, there’d be an occasional spike again, but as I looked at the daily positivity rate, it was steady and getting lower. The news always mentions how many tests were conducted the previous day. I feel like a doofus for not noticing it earlier, but I finally noticed that some of the spikes were on days when there had also been a sudden jump for whatever reason in the number of tests carried out. Yet a big chunk of provincial planning around re-opening was based partly on the number of shots in arms, but also the daily cases. Which now felt to me like they were missing the point. One day the testing goes up 70% and the day’s case number goes up only 20%…that’s a good thing, despite the hyperbolic “we had a jump in case numbers.”

This feels like a Rant of the Obvious. But I guess this is my question to those better-versed in epidemiology than I: should the media be focusing more on positivity rates than actual new cases? Would that give a more accurate indication of the current spread of the virus in the province? Is it media laziness (duh) that a solid number is an easier sell than a percentage of an ever-changing sample size for a news story?

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While the general point is apt, it’s worth noting that usually large spikes in testing are usually the result in large spikes in numbers of people who have a reason to get tested, either due to symptoms or learning they’ve been in contact with a confirmed case. So, if the reason that testing is up 70% is that there’s been a spike of people with known exposure to covid, that’s not a great thing - though if there’s only been a 20% increase in cases then it’s not a horrible thing either.

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I think you are meaning “short shrift”? :slightly_smiling_face:

Anyway, I think the positivity rate is also somewhat misleading, in that, as you point out, it is dependent on the number of tests being performed, which varies daily. However, IMHO the positivity rate is more an indicator of how well the virus is spreading compared to new cases. However, I also think the number of hospitalizations, ICU bed utilization, and deaths due to COVID-19 are more important indicators of how well a community is handling things.

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All of those have been dropping as well around here. We’re doing fairly well. I totally expect a seasonal jump and a possible backsliding on how much is open come fall, but them’s the breaks. I just found myself feeling a lot of “That’s not how math works!” when I read the daily updates, when there was a jump in case numbers but a legit drop in the positivity rate. I don’t like worrying about the wrong things. Like my horrible malapropisms in thread titles, fr’example.