Data Deficient is a category applied by the IUCN, other agencies, and individuals to a species when the available information is not sufficient for a proper assessment of conservation status

Conservation status

The conservation status of a group of organisms indicates whether the group is still extant and how likely the group is to become extinct in the near future...

 to be made. This does not necessarily indicate that the species has not been extensively studied; but it does indicate that little or no information is available on the abundance

Abundance (ecology)

Abundance is an ecological concept referring to the relative representation of a species in a particular ecosystem. It is usually measured as the large number of individuals found per sample...

 and distribution of the species. The IUCN recommends that care be taken to avoid classing species as Data Deficient when the absence of records may actually indicate dangerously low abundance: "If the range of a taxon is suspected to be relatively circumscribed, if a considerable period of time has elapsed since the last record of the taxon, threatened status may well be justified" (see also precautionary principle

Precautionary principle

The precautionary principle or precautionary approach states that if an action or policy has a suspected risk of causing harm to the public or to the environment, in the absence of scientific consensus that the action or policy is harmful, the burden of proof that it is not harmful falls on those...

).