A Meta-Analytic Review of Single-Case Experimental Studies on Vocabulary Interventions for Young Children with Language Disorders: Effect Sizes and Qualitative Indicators

Objectives: Vocabulary serves as a foundational component for language comprehension and expression and plays a crucial role in communication and literacy for preschoolers and early school aged children. This study aimed not only to systematically analyze and conduct a meta-analysis of single-case experimental studies on vocabulary interventions for children with language disorders, but also to examine quality indicators to evaluate the evidence-based nature of the interventions and to provide implications for future studies. Methods: Ten single-case design studies published in peer-reviewed journals from 2010 to 2024 were selected based on predefined inclusion criteria. Each study was analyzed in terms of research and intervention characteristics, and independent and dependent variables. Effect sizes were calculated using the percent of non-overlapping data (PND) and Tau-U. Research strength was assessed using the quality indicators proposed by Reichow et al. (2008). Results: The analysis revealed that the majority of studies demonstrated large to very large effect sizes, confirming the overall effectiveness of vocabulary interventions regardless of disability type or intervention method. In contrast, significantly greater effect sizes were observed in early school-age children compared to preschoolers, and in vocabulary comprehension relative to expression. However, no studies met the ‘strong’ level, with only two rated as ‘adequate’ and eight as ‘weak.’ Major limitations included insufficient reporting of participant characteristics, lack of inter-observer agreement data, limited assessment of treatment fidelity, and inadequate consideration of social validity. Conclusion: While vocabulary interventions were generally effective for children with language disorders, most studies fell short of meeting methodological quality standards. Future research should enhance methodological validity and adhere to established quality indicators to strengthen the scientific evidence base for clinical practice.