Glossary

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ABA Reversal Method(also: ABA Design, Reversal Design)

The ABA reversal method is a single-subject experimental design in which one participant is observed across three phases: a baseline (A), an intervention (B), and a return to baseline (A). By comparing performance across the A-B-A sequence, the design isolates the effect of the…

Ability Heuristics

A set of nine accessibility-focused design heuristics — Adaptability, Equitable Experience, Flexible Task Completion, Efficiency and Effectiveness of User Action, Multiple Modalities, Understandable Messages, Ease of Adoption, Ability Data Transparency, and Help, Support, and…

Accessibility Barrier(also: A11y Barrier)

Any aspect of a digital product, web page, document, or service that prevents or impedes a person with a disability from perceiving, operating, understanding, or using it on an equivalent basis to someone without that disability. Examples include missing alt text on images,…

Accessibility Inspection(also: Accessibility Inspection Method, Accessibility Audit)

An evaluation approach in which an expert or designer reviews an interface against a set of accessibility criteria without recruiting end users, analogous to usability inspection methods such as heuristic evaluation, cognitive walkthrough, or guideline review. Common inspection…

Accessibility Internet Rally(also: AIR, AIR-Austin)

An annual web-development competition run by Knowbility (based in Austin, Texas) in which teams of developers are paired with non-profit clients and judged partly on the accessibility of the websites they build in a short timeframe. AIR uses a structured judging rubric that…

Algorithmic Audit(also: AI Audit, Algorithmic Auditing)

A structured evaluation of an algorithmic system that measures how its behaviour differs across users, groups, or contexts - typically to surface bias, fairness failures, or disparate impact. Accessibility-oriented audits go beyond aggregate accuracy to look at where and why a…

Barrier Impact Factor(also: BIF, Accessibility Barrier Impact)

A metric for measuring the severity of web accessibility barriers by weighting detected errors according to which assistive technologies and disability groups they affect. The BIF is calculated by summing the product of the number of errors and a weight value for each affected…

Checkpoint(also: WCAG Checkpoint, Success Criterion, Accessibility Checkpoint)

A checkpoint is a specific, testable accessibility requirement in a set of guidelines — for example, 'provide a text equivalent for every non-text element' is WCAG 1.0 checkpoint 1.1. The term is strongly associated with WCAG 1.0, which was organised into 65 numbered checkpoints…

Dunning-Kruger Effect

A cognitive bias in which people with limited knowledge or competence in a domain overestimate their ability, while those with greater expertise tend to underestimate their relative skill. In accessibility, the Dunning-Kruger effect appears when developers or designers believe a…

Error Profile(also: Accessibility Error Profile, Violation Profile)

An error profile is a structured summary of the accessibility issues detected on a page, typically represented as a numeric vector with one component per checkpoint or rule — counts of violations, binary pass/fail indicators, or failure rates. Error profiles were introduced in…

Failure Mode(also: Accessibility failure mode)

In accessibility evaluation, any hindrance caused by a product or web site that prevents a user with a disability from achieving a goal with the same effectiveness, efficiency or safety as a non-disabled user. The term is borrowed from reliability engineering and is used to…

Failure Rate(also: FR (accessibility metric))

An accessibility metric introduced by Sullivan and Matson (2000) that, for a given page and a given checkpoint, divides the number of checkpoint violations found by the maximum number of violations that could have occurred on that page. Failure Rate produces a normalised value…

Functiona11ity Error(also: Functionality Error, Functiona11ity)

A term coined by Zhong et al. (2026) — a portmanteau of 'functionality' and 'a11y' (accessibility) — for an accessibility barrier that only manifests through interaction. The static state of the UI appears accessible (elements have labels, sufficient contrast, correct roles),…

Heuristic Walkthrough(also: Heuristic walk-through)

A usability evaluation method proposed by Andrew Sears (1997) that combines scenario-based cognitive walkthrough with heuristic evaluation. Evaluators work through realistic user tasks using a prioritised list of heuristics, surfacing both task-specific and general usability…

Lighthouse(also: Google Lighthouse, Chrome Lighthouse)

An open-source automated tool developed by Google for auditing web page quality, including performance, accessibility, SEO, and progressive web app compliance. Lighthouse runs in Chrome DevTools, as a browser extension, or via command line, and produces scores from 0-100 for…

Listenability(also: Auditory Readability, Speech-Output Quality)

A web-accessibility usability metric that measures how appropriate a page's rendered text is when read aloud by a screen reader or voice browser — complementary to, and distinct from, raw WCAG conformance. Listenability penalises meaningless or placeholder ALT text (such as…

Navigability(also: Ease of Navigation, Web Navigability)

The ease and efficiency with which a user can move through a web page, application, or document to reach their intended content. For accessibility practice, navigability is a primary determinant of whether a screen-reader, voice-browser, or keyboard-only user can actually…

OPTIMAL-EM(also: Optimised Evaluation Methodology)

A web accessibility evaluation methodology proposed by Hambley, Yesilada, Vigo, and Harper to complement the W3C's WCAG-EM by providing a statistically grounded, complexity-driven method for selecting representative pages from a large website. OPTIMAL-EM comprises six metrics —…

Random Walk(also: Random Walk Sampling)

In web-accessibility evaluation, a random walk is a probabilistic sampling method that starts from a seed page (typically the home page) and follows outgoing links according to a probability rule — for example, with probability d follow a uniformly-chosen outgoing link, and with…

Representative Sampling(also: Representative Page Sampling)

In web accessibility auditing, the practice of selecting a subset of pages from a website that statistically reflects the full site, so that evaluation findings can be generalised to pages not directly audited. WCAG-EM requires that a 'representative sample' be included…

Sampling Method(also: Sampling, Page Sampling, Site Sampling)

A sampling method is a rule for selecting a subset of pages, screens, or components from a larger product to be evaluated for accessibility. Because full manual evaluation of every page on a real-world website, app, or PDF library is usually infeasible, every practical audit…

Small-n Experimental Design(also: Small-sample Design, Single-case 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…

Stratified Sampling(also: Stratified Random Sampling)

Stratified sampling is a statistical technique that divides a population into non-overlapping subgroups (strata) that share some characteristic, then draws a random sample from each stratum. In accessibility evaluation, stratified sampling is used to pick test pages by first…

Task Analysis(also: Hierarchical Task Analysis, HTA)

A systematic method for breaking down complex activities into their component tasks, subtasks, and actions to understand how users accomplish goals when interacting with a system. In accessibility and usability evaluation, task analysis is used to identify potential barriers by…

Universal Access Reference Model(also: UARM)

A conceptual framework for understanding and addressing the full range of user needs in information and communication technology. The Universal Access Reference Model provides a structured approach to identifying and removing barriers to accessibility by modelling the…

Voice Usability(also: Auditory Usability, Non-Visual Web Usability)

The degree to which a web page, application, or document is usable when accessed through a voice browser or screen reader — the audio-first counterpart to traditional visual usability. Voice usability combines structural quality (navigability — how quickly a user can reach…

Web Page Complexity(also: Page Complexity, Structural Complexity)

A measure of how much a web page contains in terms of interactivity, embedded media, and structural richness. The W3C's WCAG-EM defines complexity through three factors: level of interactivity, source and method of content generation, and implementation style. Quantitative…

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