If you want to analyze your particular state (or for friends/family), here's how to do it. First, I summed up DR's post as follows:


Number of tests needed according to positivity rate:

< 3% - 1000 tests per million people
3% - 1500 tests per million people
5% - 2000 tests per million people
10% - 2500 or 3000 tests per million people
15% - I'm not sure (those are my words, not DR's)
20% - Doesn't matter (too late)


Restrictions needed according to positivity rate:

< 3% - masks recommended at work and stores
3% - masks mandatory; limit gatherings; schools prepare to close
5% - masks mandatory; further limit gatherings
10% - close schools; shutdown except for essential business & travel until 5% reached
15% - full lockdown; no travel; no half measures
20% - full lockdown; no other choice


Real case numbers according to positivity rate:

1% - real numbers are slightly higher
3% - real numbers are significantly higher but not double
5% - real numbers are double or more (if 2000 tests per million; if less than 2000 tests, real numbers are even worse)
10% - real numbers are three times higher (if 2500-3000 tests per million; if less than 2500-3000 tests, real numbers are worse)
15% - pandemic out of control (exponential)
20% - pandemic totally out of control


Next I will post a few examples of US states. If any of the above is wrong, please correct me, DR. :)