was estimated to occur in less than 1% of the healthy
control population (two-tailed).
Patients were classified as displaying strong dissocia-
tions if they were (a) impaired at both tasks and (b)
showed a significant discrepancy across category; and
classified as displaying a classical dissociation if they
were (a) impaired in one category, but performed
normally on the other and (b) showed a significant
discrepancy between the two categories (see Crawford
& Garthwaite, 2005; Crawford, Garthwaite, & Gray, 2003).
These methods of testing for deficits and for differ-
ences (i.e., dissociations) are to be preferred over the
use of z and zD as they treat the statistics of the control
sample as statistics rather than as population parame-
ters. Moreover, Monte Carlo simulations show that the
RSDT (Crawford & Garthwaite, 2005) provides excellent
control over Type 1 error rate (regardless of whether the
data are heavily skewed, the control sample is small in
size, or the correlation between tasks). We contrasted
results from application of this method with those
derived from applying, on the same data, those derived
from using x2—the method most frequently used to
evaluate discrepancies in living/nonliving accuracy in
single cases (Laws, in press). Programs to run the RSDT
analyses can be downloaded from www.abdn.ac.uk/
%7Epsy086∕dept∕psychom.htm.
Acknowledgments
We thank Dr. Ivry and the reviewers for insights and helpful
comments on an earlier version of this article.
Reprint requests should be sent to Dr. Keith R. Laws, School of
Psychology, University of Hertfordshire, College Lane, Hat-
field, Hertfordshire AL10 9AB, UK, or via e-mail: keith.laws@
ntu.ac.uk.
Note
1. Four dementia of Alzheimer’s type patients were not test-
ed on this task.
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