A comprehensive psychometric evaluation of the Avoidance and Fusion Questionnaire for the Youth 8-item short form in a large sample of Finnish adolescents

The Avoidance and Fusion Questionnaire for Youth (AFQ-Y) and its 8-item short form (AFQ-Y8) are widely used measures of psychological inflexibility in adolescents. This study evaluated the validity, reliability, responsiveness, and interpretability of the Finnish AFQ-Y8 using an observational psychometric design with a nested RCT subsample for longitudinal analyses. The main analyses were conducted in a community sample of 885 Finnish adolescents from 15 secondary schools (aged 14-18 years; 56.2% female). To examine cross-cultural validity (measurement invariance testing), data from 246 Spanish adolescents (48.4% female) and 152 Portuguese adolescents (60.5% female) from previous AFQ-Y8 validation studies were also included. Confirmatory factor analysis (CFA; WLSMV estimator), multi-group CFA for measurement invariance (configural, metric, scalar, and strict) across gender, depression status, time, and countries, correlations with related constructs (depression, stress, mindfulness, and life satisfaction), ROC curve analyses, reliability indices (including 7-week test-retest), and the estimation of clinically useful change indices were computed. CFA supported a robust single-factor structure (CFI = .989, TLI = .985, RMSEA = .069, 90% CI [.056, .083]) and broad measurement invariance, with partial strict invariance across Finland, Spain, and Portugal. AFQ-Y8 scores correlated strongly with symptoms of depression and stress, mindfulness, and life satisfaction. The scale also demonstrated good internal consistency (α = .89, ω = .89), satisfactory test–retest reliability (ICC = .84), and promising utility for identifying depression caseness and interpreting meaningful change. These findings support the use of the AFQ-Y8 to identify and monitor psychological inflexibility in adolescents, a core process targeted in ACT.