This paper describes the process and the technical background to the IUCN Red List Categories and Criteria. The IUCN system had to be both broad and flexible to be applicable to the majority of described species, taking into account the major differences between species, the range of threatening processes affecting them, and the limited knowledge available for most species. The system was designed to measure the symptoms of extinction risk, and uses five independent criteria relating to aspects of population loss and decline of range size.
Mace, G.M., Collar, N.J., Gaston, K.J., Hilton-Taylor, C., Akçakaya, H.R., Leader-Williams, N., Milner-Gulland, E.J. and Stuart, S.N. 2008. Quantification of extinoction risk: IUCN's system for classifying threatened species. Conservation Biology, 22(6): 1424-1442. DOI: 10.1111/j.1523-1739.2008.01044.x