If about 100 people in San Francisco had the virus, there would be about a 10 percent chance that a 1,000-person gathering includes an infected person.

Judged as of 2020-03-13, as a calculation. Under the stated assumptions (100 infected among about 880,000 residents, random mixing, a 1,000-person office), the probability that at least one person in the office is infected is 1 − (1 − 100/880,000)^1000, which is about 10.7%. True if the correct calculation gives roughly 9–12%. Mostly true if it gives roughly 7–9% or 12–15%. Mixed if it gives 5–7% or 15–20%. Mostly false if it gives 3–5% or 20–30%. False if it falls outside those bounds.

Set before the research began.

1. E1 Milpitas, Tracy Lead Bay Area Population Growth Silicon Valley One World · 26 June 2020

   “San Francisco +0.1 percent 881,549”

2. E2 Simple mathematical modelling approaches to assessing the transmission risk of SARS-CoV-2 at gatherings PubMed Central (peer-reviewed journal article by other authors) · 7 May 2021

   “Introduction risk can be broadly assessed with the population prevalence and the size of the gathering, while transmission risk at a gathering is mainly driven by the gathering size.”

E1 puts San Francisco's population at 881,549, which matches the stated figure of about 880,000. Under random mixing, the chance that a 1,000-person office has at least one infected person is 1 − (1 − 100/880,000)^1000 ≈ 10.7%. That is inside the 9–12% band the criteria set for true. E2 backs the method: an independent paper says the chance an infected person is present depends on how common the infection is in the population and on the size of the group.