The Washington Psychosocial Seizure Inventory (WPSI): psychometric evaluation and future applications.

The Washington Psychosocial Seizure Inventory (WPSI) clinical scales were developed via an empirical item selection approach and have been used widely to measure aspects of psychosocial functioning of patients with epilepsy. However, these empirically derived clinical scales have not been assessed psychometrically using a modern item response theory-based model. The goals of this study were to: (1) evaluate how items in each clinical scale performed in such a way as to represent the underlying constructs being measured; and (2) derive a shorter version while maintaining measurement precision. WPSI item response data from 145 adults with epilepsy collected for an evaluation study of an intervention to pact negative attributional style in epilepsy were used. The dichotomous Rasch model suitable for the true-false response choices was used to analyse each clinical scale separately. Most items within each scale fit the measurement model well, with very few exceptions. All items, therefore, were retained. A method, based on computerised adaptive testing (CAT), is offered for shortening the WPSI using a psychosocial outcomes item bank derived from the study. Individuals' specific levels of functioning are used to derive measures of their psychosocial functioning with a minimum number of items.