Assessment of selective mutism (SM) is hampered by the lack of diagnostic measures. The Frankfurt Scale of Selective Mutism was developed for kindergarteners, schoolchildren, and adolescents, including the diagnostic scale (DS) and the severity scale (SS). The objective of this study was to evaluate this novel, parent-rated questionnaire among individuals aged 3 to 18 years (n = 334) with SM, social phobia, internalizing disorders, and a control group. Item analysis resulted in high item-total correlations, and internal consistency in both scales was excellent with Cronbach's α = .90-.98. Exploratory factor analysis of the SS consistently yielded a one-factor solution. Mean sum scores of the DS differed significantly between the diagnostic groups, and the receiver operating characteristic analysis resulted in optimal cutoffs for distinguishing SM from all other groups with the area under the curves of 0.94-1.00. The SS sum scores correlated significantly with SM's clinician-rated symptom severity.
Keywords: measure development/validation; psychometric properties; reliability; selective mutism; validity.