IUCN Red List Guidelines - The IUCN Red List of Threatened Species

Red List Guidelines 18
3.2.3 Dispute tolerance and risk tolerance
When interpreting and using uncertain data, attitudes toward risk and uncertainty are
important. First, assessors need to consider whether they will include the full range of
plausible values in assessments, or whether they will exclude extreme values from
consideration (known as dispute tolerance). Uncertainty in the data is reduced when an
assessor has a high dispute tolerance, and thus excludes extreme values from the assessment.
We suggest assessors adopt a moderate attitude, taking care to identify the most likely
plausible range of values, excluding extreme or unlikely values.
Second, assessors need to consider whether they have a precautionary or evidentiary attitude
to risk (known as risk tolerance). A precautionary attitude (i.e., low risk tolerance) will
classify a taxon as threatened unless it is highly unlikely that it is not threatened, whereas an
evidentiary attitude will classify a taxon as threatened only when there is strong evidence to
support a threatened classification.
3.2.4 Dealing with uncertainty
It is recommended that assessors should adopt a precautionary but realistic attitude, and to
resist an evidentiary attitude to uncertainty when applying the criteria (i.e., have low risk
tolerance). This may be achieved by using plausible lower bounds, rather than best
estimates, in determining the quantities used in the criteria. It is recommended that ‘worst
case scenario’ reasoning be avoided as this may lead to unrealistically precautionary listings.
All attitudes should be explicitly documented. In situations where the spread of plausible
values (after excluding extreme or unlikely values) qualifies a taxon for two or more
categories of threat, the precautionary approach would recommend that the taxon be listed
under the higher (more threatened) category.
In some rare cases, uncertainties may result in two non-consecutive plausible threat
categories. This may happen, for example, when extent of occurrence (EOO) or area of
occupancy (AOO) is smaller than the EN threshold and one subcriterion is definitively met,
but it is uncertain whether a second subcriterion is also met. Depending on this, the category
can be either EN or NT. In such cases, the category could be specified as the range EN–NT
in the documentation (giving the reasons why), and the assessors must choose the most
plausible of the categories, of which VU could be one. This choice depends on the level of
precaution (see section 3.2.3) and should be justified.
Specific guidelines for dealing with uncertainty in assessing taxa with widely distributed or
multiple subpopulations against criterion A are given in section 5.8. This section offers clear
guidance on using uncertain estimates, uncertainty about the pattern of population decline
and using data with different abundance units.
3.2.5 Documenting uncertainty and interpreting listings
The level of uncertainty associated with a particular taxon’s assessment is not apparent from
the listing itself, potentially complicating and de-valuing interpretation of listings. When a
plausible range for each quantity is used to evaluate the criteria, a range of categories may be