When Can We Improve Subscores by Making Them Shorter?: The Case Against Subscores with Overlapping Items

Subscores can be of diagnostic value for tests that cover multiple underlying traits. Some items require knowledge or ability that spans more than a single trait. It is thus natural for such items to be included on more than a single subscore. Subscores only have value if they are reliable enough to justify conclusions drawn from them and if they contain information about the examinee that is distinct from what is in the total test score. In this study we show, for a broad range of conditions of item overlap on subscores, that the value of the subscore is always improved through the removal of such items.