608 Neurocognitive Disorders
in Table 1, can also supply objective data in settings where such testing is unavailable or
infeasible. In any case, as with cognitive co ncerns, objective perfor mance must be inter-
preted in light of the individual’s prior pe rformance. Optimally, this information would
be available from a prior administration of the same test, but often it must be inferred
based on appropriate norms, along with the individual’s educational history, occupation,
and other factors. Norms are more challenging to interpret in individuals with very high
or very low levels of education and in indivi duals being tested outside their own language
or cultural background. 
Criterion B relates to the individual’s leve l of independence in everyday functioning.
Individuals with major NCD will have impairment  of sufficient severity so as to interfere
with independence, such that others will have to take over tasks that the individuals were
previously able to complete on their own. Individuals with mild NCD will have preserved
independence, although there may be subtle in terference with function or a report that
tasks require more effort or take more time than previously. 
The distinction between major and mild NCD is inherently arbitrary, and the disorders
exist along a continuum. Precis e thresholds are therefore difficult to determine. Careful
history taking, observation, and integration wi th other findings are required, and the im-
plications of diagnosis should be considered when an individual’s clinical manifestations
lie at a boundary.
Associated Features Supporting Diagnosis
Typically the associated features that suppo rt a diagnosis of major or mild NCD will be
specific to the etiological subtype (e.g., neur oleptic sensitivity and visual hallucinations in
NCD due to Lewy body disease). Diagnostic features specific to each of the subtypes are
found in the relevant sections.
Prevalence