OPTIMAL-EM: Complexity-Driven Clustering for Optimised Web Accessibility Evaluation

The Website Accessibility Conformance Evaluation Methodology (WCAG-EM) directs an auditor to select samples of web pages for accessibility evaluation. However, the sampling method of WCAG-EM cannot statistically claim to represent pages outside of the selected sample. To generate an optimal representative sample, we previously proposed OPTIMAL-EM. In this paper, we apply the framework to explore the relationships between two metrics in the framework: web page complexity and accessibility. We first cluster pages by their structure by employing t-distributed Stochastic Neighbour Embedding (t-SNE) and Density-Based Spatial Clustering of Applications with Noise (DBSCAN) by representing pages using two approaches: one including all HTML elements and another focusing on block-level HTML elements. We then compare the average complexity of a cluster and the variance of the complexity of a given cluster against the average accessibility of that cluster. To measure their correlations, we conducted a two-stage evaluation: an initial experiment with a website of 388 pages and a validation study with three additional random sites of 500 pages. Our experiments demonstrate that clusters characterised by lower variability in complexity tend to exhibit fewer accessibility barriers, suggesting that standardised and templated web design could be more beneficial to accessibility than more complex pages. Furthermore, the main contribution of this paper is showing that by identifying and focusing on clusters with high complexity variance, auditors can target their efforts more effectively, prioritising areas more likely to present accessibility challenges. By providing a more systematic and scalable sampling method, our approach can optimise conformance evaluations of large websites by reducing the human effort involved, focusing on a more targeted assessment of more representative pages.