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Abstract
Purpose. To apply Rasch analysis to the Beck Depression Inventory-II (BDI-II) responses of a mixed neurorehabilitation sample to determine its suitability for assessing depression among this group.
Method. Three hundred and fifteen in-patients undergoing post-acute specialist in-patient rehabilitation at the Regional Rehabilitation Unit (RRU) of Northwick Park Hospital in North London were administered the Beck Depression Inventory. All patients were administered the BDI-II by a clinical psychologist using a large print version and taking as much time as patients required. Rasch analysis was completed using the RUMM2020 software. Where disordered thresholds were identified item rescoring was undertaken. Each item was also examined for Differential Item Functioning. Where misfit to model expectations was identified items were removed in an iterative fashion. The effect of any deletion upon person estimates was examined. Specific tests of unidimensionality were undertaken at each iterative phase.
Results. Rescoring was necessary before Items 1–4, 6, 8–12, 20 and 21 showed ordered thresholds. Three items failed to satisfy model expectations – items 16 (changes in sleeping pattern), 18 (changes in appetite) and 21 (loss of interest in sex) and were deleted – after which good overall fit to the Rasch model was observed. The three items deleted affected person estimates in a significant way such that the 21 item version in this group of patients may be biased because of multidimensionality.
Conclusions. The BDI-II in a sample of patients undergoing neurorehabilitation satisfies unidimensional Rasch model expectations in an 18-item format. Some disordering of response categories was evident, but this evidence requires replication before any common rescoring option should be considered.
Acknowledgements
We are grateful to all the patients on the Regional Rehabilitation Unit, Northwick Park Hospital, who allowed their data to be used and the staff who performed the assessments. We are especially grateful to Dr. Frances Clegg for initiating the dataset and to Margot Kalmus, Jo Clark and Heather Williams for their assistance with collation and validation of the dataset. Financial support for the preparation of this manuscript was provided by the Dunhill Medical Trust and the Luff Foundation. We would also like to thank two anonymous reviewers for their helpful suggestions.