RESEHARCH PAPER IN THE UNITED STATES
Quantifying
Dissociations in Neuropsychological Research
This study
introduces a statistical procedure to determine the probability
of a double dissociation when the correlation
between measures is taken into account.
Different
quantitative
definitions
of dissociations were compared in two large samples of neurological patients,
and applied to four pairs of
measures (two for language, two for hemispatial neglect) with different degrees
of intercorrelation (ranging
from þ.21 to þ.84). If the correlation between measures is not taken into
account, large numbers of dissociated
cases may be missed, especially for measures that are highly correlated. There
are also qualitative differences between methods in the identity of
those individuals who meet each definition.
Double
dissociations play an important role in neuropsychology, but they are often
identified through
subjective
estimates of ‘‘high’’ versus ‘‘low’’ performance, without considering the
probability that such an outcome
might have occurred by chance. To determine whether two measures ‘‘come apart’’
in an interesting way in
brain-damaged patients, it is important to know the degree to which variance in
one measure can be predicted by variance in the other.
