A Note on Specifying the Guessing Parameter in ATFIND and DIMTEST
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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Socha, A. & DeMars, C. E. (2013). A note on specifying the guessing parameter in ATFIND and DIMTEST. Applied Psychological Measurement, 37, 69-74