The indicator measures the proportion of known species for which extinction risk has been assessed through the IUCN Red List of Threatened Species. The IUCN Red List itself has a >50 year history, with its underlying methodology robustly published in the scientific literature (Mace et al. 2008 Conserv. Biol.; IUCN 2012 https://portals.iucn.org/library/node/10315). The indicator of “proportion of known species assessed through the IUCN Red List” has been tracked for many years through the summary statistics updated several times annually on the IUCN Red List website (http://www.iucnredlist.org/about/summary-statistics) (Lacher & Hilton-Taylor, 2017).