The psychometric properties of the Nepean Belief Scale as a tool for assessing delusions in schizophrenia and related psychotic disorders.

Objectives: To examine the psychometric characteristics of the Nepean Belief Scale (NBS) in psychiatric inpatients with delusions. The NBS is a five-item, clinician-administered scale that assesses the characteristics of beliefs, i.e. conviction, fixity, fluctuation, resistance and awareness that the belief is unreasonable.

Methods: Fifty-five patients were interviewed by two clinicians, within three days of admission to an acute psychiatric unit and were assessed using the NBS, the Brown Assessment of Belief Scale (BABS), the MINI International Neuropsychiatric Interview (MINI) and the Depression Anxiety Stress Scale 21-Item Version (DASS-21). The NBS was administered after two weeks to available participants, to assess test-retest reliability.

Results: Results demonstrated excellent inter-rater reliability of 0.93, Cronbach's alpha coefficient for internal consistency was 0.77. The NBS was found to have good convergent validity with the BABS and good discriminant validity with the DASS. Two-week test-retest reliability suggests that the NBS is sensitive to therapeutic change.

Conclusions: Advantages of the NBS include its brevity, its ability to assess belief-related insight, its clear instructions and its definitions of belief characteristics. Thus, the NBS has the potential to greatly improve our ability to more objectively assess delusional beliefs.