The Beck Depression Inventory-II (BDI-II; Beck, Steer, & Brown, 1996) is intended to measure severity of depression, and because items represent a broad range of depressive symptoms, some multidimensionality exists. In recent factor-analytic studies, there has been a debate about whether the BDI-II can be considered as one scale or whether subscales should be distinguished. In the present study, we applied a bifactor model to evaluate the extent to which scores reflect a single variable in a large sample of 1,530 clinical outpatients. We found that total scale score variation reflected some multidimensionality, but not enough to justify the scoring of subscales. We conclude that the BDI-II total scale score reflects a single construct and that reporting and interpreting subscale scores may result in misleading conclusions.

The Dispositional Hope Scale (DHS; C. R. Snyder et al., 1991) consists of two subsets of items measuring Agency and Pathways. The authors used bifactor analysis to evaluate the dimensionality structure of the scale. Data from 676 persons (295 psychiatric patients, 112 delinquents, and 269 students) were analyzed. The authors conclude that although the Pathway items seem to explain some additional variance when the Hope scale variance is partionalized out, the DHS allows unidimensional measurement.

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.

Purpose The St George's Respiratory Questionnaire (SGRQ) has clearly acquired the status of legacy questionnaire for measuring health-related quality of life in patients with chronic obstructive pulmonary disease (COPD). The main aim of this study was to assess the underlying dimensionality of the SGRQ and to investigate the added value of the empirical weights used to calculate total scores. Methods The official Dutch translation of the SGRQ was completed by 444 COPD patients participating in two clinical studies. These data were used for secondary data analysis in this study. Three complementary statistical methods were used to assess dimensionality: Mokken scale analysis (MSA), parametric multidimensional item response theory (IRT) and bifactor analysis. Additionally, the original SGRQ weighting procedure was compared to IRT-based weighting.Results The results of the MSA and multidimensional item response theory (MIRT) pointed toward a unidimensional structure. The bifactor analyses indicated that there was a strong general factor, but the group factors did have additional value. Nineteen items performed poorly in the MSA, MIRT analysis or both. Shortening the scale from 50 to 31 items did not negatively impact measurement precision. SGRQ total score and IRT-derived scores correlated strongly, 0.90 for the one-parameter model and 0.99 for the two-parameter model. Conclusion The SGRQ contains some multidimensionality, but an abbreviated version can be used as a unidimensional tool in patients with COPD. Subscale scores should be used with care. SGRQ total scores correlated highly with IRT-based scores, and thus, the weighting methods may be used interchangeably to calculate total scores.