OBJECTIVE: Single-item questionnaire assessments may be summarily dismissed by some, but it is unclear if this is warranted. We suggest here that the answer to that question depends on their reliability and construct validity. Reliability of a single-item index cannot be assessed via indices like coefficient alpha, but it can be addressed otherwise
construct validity can be assessed as it would be with any other quantitative index. METHODS: In 489 patients with mood and anxiety disorders, we examined the Beck Depression Inventory Item 9's (BDI-Item 9) test-retest coefficient, and its validity, in part via invariance analyses. More specifically regarding invariance analyses, we determined whether a covariance structure including a multi-item assessment of suicidal ideation was invariant with a similar structure using a single-item assessment of suicidal ideation instead. RESULTS: Reasonably supportive evidence emerged for the reliability and construct validity of this specific one-item index. CONCLUSIONS: The BDI-Item 9 can reliably assess suicidal ideation when data lack multi-item assessments.