What useful information does positivity rate give us given the fact that there is no consistent standard as to who gets tested?
There are two numbers that are looked at typically, total new cases and percent positive. If you just report cases, you can get into issues such as NY seeming to have fewer cases than FL because fewer tests were administered when NY had its wave than FL. So by combining that with positivity rate, we have a clearer idea of how widespread infection truly is. When NY is reporting 1,000 new cases but 50% of people testing are positive, it’s safe to assume that there is a very high rate of cases in the city that are not being reported/tested (which is what happened here in March). When FL reports 5,000 new cases a day but only 15% of people testing are positive, it’s safe to assume that there isn’t nearly as a high a rate of undetected cases in the community.
When there is a steady number of tests being administered, and total cases and/or positivity increases, you can be sure there is a rising number of infections relative to those that remained steady.