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Glossary

Terms used in accessibility research and practice. Each entry has a definition, common aliases, and category tags.

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Index of Difficulty(also: ID, Fitts ID)
The Index of Difficulty (ID) is the central quantity in Fitts' law that captures how hard a rapid aimed pointing movement is, computed as log₂(A/W + 1) in the Shannon formulation, where A is the amplitude (distance to the target) and W is the target width along the movement…
Internal Reliability(also: Internal Consistency)
A psychometric property that measures whether all items in a questionnaire or instrument contribute consistently to the overall score. It is commonly assessed using Cronbach's alpha, where values of 0.7 and above are generally considered acceptable. In accessibility research,…
Interpersonal Reactivity Index(also: IRI)
A widely used multidimensional self-report measure of empathy developed by Mark H. Davis in 1980. The instrument contains four seven-item subscales: perspective taking (the tendency to adopt another's point of view), empathic concern (feelings of warmth and compassion for…

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