This study investigated the performance of Poly-DIMTEST (PD) to assess
unidimensionality of test data produced by polytomous items. Two type
s of polytomous data were considered: (1) tests in which all items had
the same number of response categories, and (2) tests in which items
had a mixed number of response categories. Test length, sample size, a
nd the type of correlation matrix (used in factor analysis for selecti
ng AT1 subset items) were varied in Type I error analyses. For the pow
er study, the correlation between Bs and the item-theta loadings were
also varied. The results showed that PD was able to confirm unidimensi
onality for unidimensional simulated test data, with the average obser
ved level of significance slightly below the nominal level. PD was als
o highly effective in detecting lack of unidimensionality in various t
wo-dimensional tests. As expected, power increased as the sample size
and test length increased, and the correlation between the theta s dec
reased. The results also demonstrated that use of Pearson correlations
to select ATI items led to equally good or better performance than us
ing polychoric correlations; therefore Pearson correlations are recomm
ended for future use.