Measuring irritability across childhood, adolescence, and young adulthood: an investigation of measurement invariance by age, sex, and informant.

Irritability is a transdiagnostic phenomenon, frequently present in youth mental health services, and associated with multiple adverse outcomes. Irritability research suggests developmental, sex, and informant differences, but robust assessment requires comparison of the same or equivalent measures. Studies on irritability measurement invariance (equivalence) across age, sex, and informant remain limited. This research investigated measurement invariance across age, sex, and informant (parent/self), in the Avon Longitudinal Study of Parents and Children. Primary analyses assessed age (N = 9,809) and sex (N = 9,803) invariance in irritability measured using the parent-rated Development And Well-Being Assessment (DAWBA) at ages 7-25, and informant invariance (parent- versus self-report) was tested at age 25, separately for males (N = 2,494) and females (N = 3,408). We did not find evidence of even weak (metric) invariance across age, indicating that the DAWBA-measured irritability may not be developmentally comparable. Findings by sex were mixed, with strict invariance suggested at age 13, strong at age 7, weak at age 25, and not even weak at ages 10 and 15, suggesting that the DAWBA may not always capture the same irritability construct across sex. Informant invariance levels were measure- specific. The DAWBA demonstrated strict invariance across informants for both males and females, suggesting equivalent interpretation of irritability items and similar residuals at age 25 for self- and parent-reports. However secondary informant analyses, assessed separately for males (N = 2,505) and females (N = 3,415) in the Affective Reactivity Index (ARI) at age 25, did not support even weak invariance for males, but found evidence of strict invariance for females. Our findings suggest that for the DAWBA, comparisons of mean irritability scores are likely valid across parent-/self-reports at age 25, but incomparable across age and sex; for the ARI, parent-/self-reports may be comparable at age 25 for females, but not males.