Small-n Experimental Design

Small-n (or single-case) experimental design is a family of research methodologies aimed at drawing rigorous causal conclusions from very few — sometimes just one — participants. The approaches include ABA reversal, multiple baseline designs, alternating treatments, and changing criterion designs, typically coupled with repeated measurements over time and non-parametric statistical analyses such as randomization tests. Small-n designs are central to accessibility research because the populations of interest are often small or heterogeneous — people with rare conditions, specific combinations of impairments, or particular assistive-technology expertise — and traditional between-subjects statistical approaches lack power under those conditions. The classical practitioner reference is Todman and Dugard's 'Single-case and Small-n Experimental Designs: A Practical Guide to Randomization Tests'.