A Note on Assessing the Added Value of Subscores



Brennan (2012) noted that users of test scores often want (indeed, demand) that subscores be reported, along with total test scores, for diagnostic purposes. Haberman (2008) suggested a method based on classical test theory (CTT) to determine if subscores have added value over the total score. According to this method, a subscore has added value if the corresponding true subscore is predicted better by the subscore than by the total score. In this note, parallel-forms scores are considered. It is proved that another way to interpret the method of Haberman is that a subscore has added value if it is in better agreement than the total score with the corresponding subscore on a parallel form. The suggested interpretation promises to make the method of Haberman more accessible because several practitioners find the concept of parallel forms more acceptable or easier to understand than that of a true score. Results are shown for data from two operational tests.