Background: The Zarit Burden Interview ( ZBI) was originally developed to assess the level of subjective burden in caregivers of people with dementia. The Involvement Evaluation Questionnaire ( IEQ) is amongst the leading scales to assess caregiving consequences in severe mental illness. We aimed to compare the psychometric properties of the ZBI, a generic tool, and of the IEQ, a more specific tool to assess the consequences of caregiving in schizophrenia and related disorders.
Methods: Secondary analyses of a 16- week, randomized controlled trial of a psychoeducational intervention in 223 primary caregivers of patients with schizophrenia or schizoaffective disorder. Psychometric properties ( internal consistency, convergent and discriminative validity, and sensitivity to change) were evaluated for both ZBI and IEQ.
Results: Internal consistency was good and similar for both scales ( ZBI: 0.91, 95% CI: 0.89, 0.94; IEQ: 0.86, 95% CI: 0. 83, 0.89). Convergent validity was relevant for similar domains ( e. g. ZBI total score vs IEQ- tension r = 0.69, 95% CI: 0. 61, 0.75) and at least moderate for the rest of domains ( ZBI total score, personal strain and role strain vs IEQ- urging and supervision). Discriminative validity against psychological distress and depressive symptoms was good ( Area Under the Curve [ AUC]: 0.77, 95% CI: 0.71, 0.83; and 0.69, 95% CI: 0.63, 0.78 - for ZBI against GHQ- 28 and CES- D respectively; and AUC: 0.72, 95% CI: 0.65, 0.78; and 0.69, 95% CI: 0.62, 0.77 - for IEQ against GHQ- 28 and CES- D respectively). AUCs against the reference criteria did not differ significantly between the two scales. After the intervention, both scales showed a significant decrease at endpoint ( p- values < 0.001) with similar standardised effect sizes for change (- 0.36, 95% CI: - 0.58, - 0.15 - for ZBI; - 0.39, 95% CI: - 0.60, - 0.18 - for IEQ).
Conclusions: Both ZBI and IEQ have shown satisfactory psychometric properties to assess caregiver burden in this sample. We provided further evidence on the performance of the ZBI as a general measure of subjective burden.