Establishing equivalence: methodological progress in group-matching design and analysis.

This methodological review draws attention to the challenges faced by intellectual and developmental disabilities researchers in the appropriate design and analysis of group comparison studies. We provide a brief overview of matching methodologies in the field, emphasizing group-matching designs used in behavioral research on cognition and language in neurodevelopmental disorders, including autism spectrum disorder, Fragile X syndrome, Down syndrome, and Williams syndrome. The limitations of relying on p values to establish group equivalence are discussed in the context of other existing methods: equivalence tests, propensity scores, and regression-based analyses. Our primary recommendation for advancing research on intellectual and developmental disabilities is the use of descriptive indices of adequate group matching: effect sizes (i.e., standardized mean differences) and variance ratios.