Interventionists interpret changes in symptoms as reflecting response to treatment. However, changes in symptom functioning and the measurement of the underlying constructs may be reflected in reported change. Longitudinal measurement invariance (LMI) is a statistical approach that assesses the degree to which measures consistently capture the same construct over time. We examined LMI in measures of anxiety severity/symptoms [i.e., Pediatric Anxiety Rating Scale (PARS), Multidimensional Anxiety Scale for Children (MASC), Screen for Child Anxiety and Related Disorders (SCARED)] in anxious youth at baseline and posttreatment. Initial fit was inadequate for 27 of 38 baseline and posttreatment models, but model modifications resulted in acceptable fit. Tests of LMI supported scalar invariance for the PARS and many, but not all, MASC and SCARED subscales. Findings suggest that the PARS, and many MASC and SCARED subscales can accurately be used to measure change over time, however, others may reflect changes in measurement properties.
