Assessing the validity and reliability of the International Anxiety Questionnaire and the International Depression Questionnaire in two bereaved national samples.

The International Anxiety Questionnaire (IAQ) and International Depression Questionnaire (IDQ) are self-report measures of ICD-11 Generalized Anxiety Disorder (ICD-11 GAD) and ICD-11 Single Episode Depressive Disorder (ICD-11 DD). This study tested the psychometric properties of these scales in two samples of bereaved adults from the United Kingdom and the Republic of Ireland. Confirmatory factor analysis (CFA) was used to test the combined dimensionality and measurement invariance of the IAQ and IDQ across the United Kingdom (n = 1012) and Irish (n = 1011) samples. Differential item functioning (DIF) was tested using multiple indicator multiple cause (MIMIC) modelling while convergent validity was also assessed. CFA results supported a correlated two-factor model in both samples. The MIMIC model showed that the IDQ item "Had recurrent thoughts of death or suicide" showed DIF and the effect was small. Internal reliability of the scales were high and convergent validity was supported. The prevalence of ICD-11 GAD was 18.6% and 16.1% and ICD-11 DD was 13.8% and 10.5% in the United Kingdom and Irish samples, respectively. Findings of the study provide support for the validity, measurement invariance, and reliability of the IAQ and IDQ among two bereaved national samples.