Providing Subscale Scores for Diagnostic Information: A Case Study When the Test is Essentially Unidimensional

Although reliability of subscale scores may be suspect, subscale scores are the most common type of diagnostic information included in student score reports. This research compared methods for augmenting the reliability of subscale scores for an 8th-grade mathematics assessment. Yen's Objective Performance Index, Wainer et al.'s augmented scores, and scores based on multidimensional item response theory (IRT) models were compared and found to improve the precision of the subscale scores. However, the augmented subscale scores were found to be more highly correlated and less variable than unaugmented scores. The meaningfulness of reporting such augmented scores as well as the implications for validity and test development are discussed.