Arizona Positive Test Percentage Downtrend Mystery Solved

 


As noted in yesterday's weekly numbers update, I have been puzzled about why the positive test percentage in Arizona has been consistently trending down when new cases have been consistently  trending up. After all, it takes a test to identify a case, so it seems like the positive test percentage should go up as the new case rate goes up.

The ASU Biodesign Dashboard has data on raw numbers of positive a negative tests.  So I downloaded that data and had a look.  

One thing to note is that the number of tests has gone way up over the last month.  On August 6 the seven-day total was about 66K tests.  By August 28 the seven-day total was about 97K tests—almost a 50% increase.

Of course, more testing don't explain the mystery downtrend by itself. But things become clearer when we look at the trends in positive vs. negative tests (dotted lines are trendlines):


As we can see, both positive and negative tests are trending upward, but negative tests are trending upward quite a bit faster than positive ones.  That explains why positive test percentage would be trending down.  The positive tests are getting lower and lower as a proportion of all tests.  But positive tests are still increasing as we would expect given the increasing case rate.

I guess I still tend to think of the tests as being a result of random sampling. But that is wrong. They are in fact driven by self-selection bias. That means people decide to get tested for their own reasons. In this case, people who aren't infected are seeking out a lot more tests. 

I'm not sure why, but here is a guess. All the talk of a new wave and these breakthrough infections is making people extra cautious. Even vaccinated people (who we know are much less likely to be infected) are seeking more tests. 

In support of this, demand for testing is high in Arizona (and across the country). The DIY home-tests are sold out at my local drugstore, and CVS is limiting how many people can buy. The test data above is not for the DIY tests, but it's an indicator of demand.

Header image is a modified version of one by OpenClipart-Vectors from Pixabay 

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