Introduction to the Special Issue. A Dozen Years of Demonstrating That Informant Discrepancies are More Than Measurement Error: Toward Guidelines for Integrating Data from Multi-Informant Assessments of Youth Mental Health.

Validly characterizing youth mental health phenomena requires evidence-based approaches to assessment. An evidence-based assessment cannot rely on a "gold standard" instrument but rather, batteries of instruments. These batteries include multiple modalities of instrumentation (e.g., surveys, interviews, performance-based tasks, physiological readings, structured clinical observations). Among these instruments are those that require soliciting reports from multiple : People who provide psychometrically sound data about youth mental health (e.g., parents, teachers, youth themselves). The January 2011 issue of the () included a Special Section devoted to the most common outcome of multi-informant assessments of youth mental health, namely discrepancies across informants' reports (i.e., ). The 2011 Special Section revolved around a critical question: Might informant discrepancies contain data relevant to understanding youth mental health (i.e., )? This Special Issue is a "sequel" to the 2011 Special Section. Since 2011, an accumulating body of work indicates that informant discrepancies often contain domain-relevant information. Ultimately, we designed this Special Issue to lay the conceptual, methodological, and empirical foundations of guidelines for integrating multi-informant data when informant discrepancies contain domain-relevant information. In this introduction to the Special Issue, we briefly review the last 12 years of research and theory on informant discrepancies. This review highlights limitations inherent to the most commonly used strategies for integrating multi-informant data in youth mental health. We also describe contributions to the Special Issue, including articles about informant discrepancies that traverse multiple content areas (e.g., autism, implementation science, measurement validation, suicide).