The article I linked embeds a Google Data Studio page:

https://datastudio.google.com/u/0/reporting/1Hl_OTbyKWVYUaEA...

In April the average percentage positive was 10.4%. Over the last 56 days since the first positive test, the average positivity rate was 12.4%, and it was greater than ten percent 62% of the time.

Even granting what you just said it is still far from your 20% claim. And your way of averaging does not make sense. For example in March there was a day when 3 tests were reported with 1 test positive for a whopping positive rate of 33%, which would contribute half a point to the number you just calculated. How is that reflective of anything meaningful? To get meaningful average, you need to total the positives and divide that by the total tests over the period of concern. You can't simply average ratios.

You're right. Neither "with about 20% coming back positive" nor "mostly hovers in single digits" precisely describes the positivitiy rate for that area over time.

Rather than debate who was more wrong about the precise positivity rate in SF, I think the more fundamental discussion is around GP's comment that there's pretty strong evidence that a "light, mostly voluntary, shelter in order" is largely sufficient to achieve the low case count and mortality seen in the Bay Area.

I think that's not only far from proven, but there are many counterpoints of areas which have been locked down hard for many weeks and yet still saw pervasive community spread, and high case and fatality rates.

CA had it's first known case on Jan 25th, and a week later Day 7 the 3rd case in CA was discovered in Santa Clara, bring the US total to 7. Day 40 / March 4th CA reports its first death. Day 43 / March 7 San Francisco bans large gatherings in city-owned facilities. By March 13th (Day 49) Santa Clara bans gatherings of 100+ people and schools close as of March 16th. March 16th, Day 52, marks the first shelter-in-place orders.

MA had it's first known case on Feb 1, the second known case was not reported until March 2. (Do we call March 2nd as Day 1 or Day 30? Lets go with 30.) By March 6 (Day 36) total cases in the State were up to 8. By March 12 (Day 42) total cases were up to 108. On March 13 (Day 43) the Boston Marathon was postponed and gatherings of 250+ were banned. On March 15th (Day 45) Baker ordered all restaurants closed, and banned gatherings over 25 people, and announced public schools would be closed starting March 17th (Day 47). On March 20th (Day 50) MA had its first reported COVID death. By March 23rd (Day 53) the governor announced a "stay-at-home advisory", closed all non-essential businesses, recommended social distancing protocols, etc.

By CA's Day 40 they had 50 cases and a reported death, while MA at their Day 40 also had about 50 positive cases (most of which contact traced to a Biogen conference) but no reported deaths. By Day 50 MA had tested 5,000 and had over 300 cases and saw their first death. CA by their Day 50 also had about 300-400 cases. Both states closed on Day 52 or Day 53 of their respective outbreak.

At the beginning both states responded approximately the same. As of today, the outbreaks are entirely distinct.

I think a better analysis would compare e.g. Boston to one specific major metro area of CA, not a whole state-by-state level comparison.

I think what is clear is that China, the initial epicenter, exported far fewer cases than Europe. This can be seen by examining countries around China and far more integrated with China and overseas Chinese communities. Really this is a disease mainly spread by close contact. So the dominant spreaders are those with high community contact. I remember the feeling of OMG when seeing the announcement of all the high flying jetsetting group getting hit in March. I am on the west coast, yet our cases didn't start going exponential until after Europe exploded. Yes Seattle and Bay Area were likely seeded from China. But it started with low count, and coupled with our sprawl and high percentage of Asians who were attuned to the seriousness of the outbreak in China, it was a simmering outbreak. The east coast otoh got a major frontal assault from Europe.