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An Assessment of the Relationship Between the Assumption of Unidimensionality and the Quality of IRT True-Score Equating IRT SAT

Cook, Linda L.; Dorans, Neil J.; Eignor, Daniel R.; Petersen, Nancy S.
Publication Year:
Report Number:
ETS Research Report
Document Type:
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Subject/Key Words:
Equated Scores, Factor Analysis, Item Response Theory (IRT), Mathematics Level II Achievement Test, Scaling, SAT


A strong assumption made by most commonly used item response theory (IRT) models is that the data are unidimensional, i.e., statistical dependence among item scores can be explained by a single ability dimension. One of the major practical applications of item response theory models to testing has been in the area of score equating. This research assesses the relationship between violations of the assumption of unidimensionality and the quality fo IRT true-score equating. Dimensionality analyses revealed that Mathematics Level II item parcels were more nearly unidimensional than SAT-verbal item parcels. In addition, the dimensionality analyses revealed that one SAT-verbal test form and one Mathematics Level II form were each less parallel to the other two forms in their respective equating chains than these other forms were to each other. Refinements in the dimensionality methodology and a more systematic dimensionality assessment are logical extensions of the present research. (72pp.)

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