IUCN’s research while affiliated with Cambridge and other places

... The RLI is based on data for a very high proportion of species in each of the taxonomic groups included, so its trends should be largely representative of those for the entire set of species in these groups [31]. This representativeness builds from the strengths of the Red List process, which is an effective way to make meaningful inferences from imprecise or incomplete data, enabling the RLI to incorporate information even from species that are rare, localized or difficult to survey [21]. For birds at least, poorly known species with less certain estimates of population size, population trend or range size do not bias the index [31]. ...

Reference:

Measuring trends in extinction risk: a review of two decades of development and application of the Red List Index