A simulation investigated use of the difficulty parameter
(Bejar, 1980) to evaluate item unidimensionality.
Artificial tests were designed to be nonequivalent
in both length and dimensionality. Simulated item responses
to the tests were analyzed with the LOGIST
computer program. Two indices were calculated: the
slope of the principal axis between the content-areabased
item difficulty estimates and corresponding total-
test-based estimates, and the correlation between
the two sets of estimates. Results show that the magnitude
of the correlation coefficient provides no information
about dimensionality of a set of test items. The
slope of the principal axis, on the other hand, is sensitive
to multidimensionality in the data as well as test
length. The size of the slope adequately detects the dimensionality
of items for relatively long tests. Index
terms: equating, item difficulty parameters, item response
Liou, Michelle. (1988). Unidimensionality versus statistical accuracy: A note on Bejar's method for detecting dimensionality of achievement tests. Applied Psychological Measurement, 12, 381-386. doi:10.1177/014662168801200406
Unidimensionality versus statistical accuracy: A note on Bejar's method for detecting dimensionality of achievement tests.
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