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Texas Governor Abbott used the "positivity rate" in his address yesterday. It's the new-cases-to-tests ratio. Abbott used the ratio as justification for the measured state re-opening already underway.
While testing is ramping up in a country or state, and the number of cases percapita ramps up you can't really tell if the increase in new cases you see represents an increase in case density or is merely the result of testing previously-hidden cases. The positivity ratio is designed to eliminate this uncertainty.
However, the ratio introduces another ambiguity as testing becomes more prevalent. You can't tell by looking at the positivity ratio how prevalent the virus is, or what prevalence trend it is taking. Does a drop in positivity mean that prevalence is dropping, or does it merely mean that testing has become so prevalent that we're testing people who have no need to be tested?
if you look at the Texas new cases per million chart it reveals a rise in new cases since the beginning of reopening. At this stage of testing prevalence I don't think the rise in cases can be attributed to the rise in testing even though it might be. The positivity drop could merely mean that we are testing people who don't need to b be tested.
Texas is over 800 miles from the panhandle border to the Brownsville border. I converse with people who point at the dropping new-cases-percapita rates of opening southern states like Florida and Georgia. They compare the rising new-cases-percapita of northern still-closed states like New York and Massachusetts as evidence that remaining closed is more dangerous.
HeellO. We know that COVID-19 is hampered by warmth, and killed by sunshine. The Southern governors in question knew this factor, and used it in deciding to reopen. Anyway, it turns out that Texas' biggest new-cases-percapita rises have been in the northern half of the state which are slower to warm up in spring.
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