Missed Connections: Fidelity and Tier Alignment in Single-Case Research Design Studies of Tier 2 Literacy Interventions

Students with and without disabilities increasingly learn to read within multitiered systems of support that provide universal (Tier 1) instruction and targeted (Tier 2) intervention. Researchers have emphasized the importance of evaluating Tier 2 interventions in relation to the quality and coherence of Tier 1 instruction. Although large-scale studies increasingly report implementation fidelity and alignment across tiers, it is unclear to what extent single-case research design (SCRD) studies include such information. The SCRD encompasses a variety of within-subject experiments that are common in special education and have potential benefits for students with or at-risk for learning disabilities. In this systematic review, we examined how SCRD studies of Tier 2 literacy interventions ( k = 9) reported instructional fidelity and alignment across tiers. While most studies documented fidelity for the implemented intervention, none described the nature or quality of Tier 1 instruction. Findings indicate that SCRD research rarely situates Tier 2 outcomes within the context of Tier 1, limiting interpretability and integration with the larger evidence base on reading interventions.