The software program DIMTEST can be used to assess the unidimensionality of item scores. The software allows the user to specify a guessing parameter. Using simulated data, the effects of guessing parameter specification for use with the ATFIND procedure for empirically deriving the Assessment Subtest (AT; that is, a subtest composed of items that potentially measure a second dimension) and DIMTEST for assessing whether that AT represents a second dimension were investigated. Results suggest that specifying higher guessing parameters in ATFIND and DIMTEST result in higher Type I error rates.
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