||P.1 right column 2nd paragraph: "Given the uncertainties in species numbers and that only a few percent of species are assessed for their extinction risk (ref 13), [investigators] express extinction rates as fractions of species going extinct over time—extinctions per million species-years (E/MSY) (ref 14)—rather than as absolute numbers. For recent extinctions, [they] follow cohorts from the dates of their scientific description (ref 15). This excludes species, such as the dodo, that went extinct before description. For example, taxonomists described 1230 species of birds after 1900, and 13 of them are now extinct or possibly extinct. This cohort accumulated 98,334 species-years—meaning that an average species has been known for 80 years. The extinction rate is (13/98,334) × 10^6 = 132 E/MSY. The more difficult question asks how [they] can compare such estimates to those in the absence of human actions—i.e., the background rate of extinction. Three lines of evidence suggest that an earlier statement (ref 14) of a “benchmark” rate of 1 (E/MSY) is too high."