Understanding the pressures that lead to a high risk of species extinction is crucial for stemming biodiversity loss (see S. L. Maxwell et al. Nature 536, 143–145; 2016). Yet the large number of species that remain classified as data deficient can introduce considerable uncertainty in identifying these drivers.

Data-deficient species represent 1 in 6 species on the IUCN Red List, or some 13,465 species. Uncertainties about drivers of extinction depend on the proportion of such species within a group (S. H. M. Butchart and J. P. Bird Biol. Conserv. 143, 239–247; 2010). In birds, uncertainty is low because just 0.58% of species are data deficient; 24% of amphibians and 49% of freshwater crabs are data deficient, so uncertainty in these groups is likely to be high.

Data-deficient species tend to have limited geographical ranges, be small in size and occupy highly specific, remote habitats (L. M. Bland et al. Conserv. Biol. 29, 250–259; 2015). They are unlikely to be threatened by hunting, but could be susceptible to deforestation and climate change, for example.

Addressing the problem of data deficiency is fundamental to diagnosing the causes of high extinction risk, and therefore to planning conservation strategies.