Measuring individual significant change on the Beck Depression Inventory-II through IRT-based statistics.

Several researchers have emphasized that item response theory (IRT)-based methods should be preferred over classical approaches in measuring change for individual patients. In the present study we discuss and evaluate the use of IRT-based statistics to measure statistical significant individual change on the Beck Depression Inventory-II (BDI-II, Beck, Steer, & Brown, 1996). We compare results obtained with a simple IRT-based statistical test (Z-test) to those obtained with the Reliable Change Index (RCI) in a sample of clinical outpatients. Mean group differences between the Z-test and the RCI were similar, but for some individuals change classifications differed. Differences were most evident for change scores within the lower range of depression scores. We show that this may have consequences for the measurement of individual change in psychotherapy outcome research and clinical practice.