Increasing the Accessibility of Pen-Based Technology: An Investigation of Age-Related Target Acquisition Difficulties
Karyn Moffatt · 2007 · SIGACCESS Accessibility and Computing · doi:10.1145/1328567.1328573
Summary
This paper describes dissertation research on improving the accessibility of pen-based devices (PDAs, Tablet PCs) for older adults by identifying and addressing the root causes of target acquisition difficulty. The author argues that previous research on target acquisition techniques has three key limitations: a narrow focus on young healthy adults, reliance on single constrained tasks, and emphasis on designing novel techniques rather than understanding basic interaction difficulties. The research adopts a three-phase approach: (1) understanding sources of difficulty through controlled experiments, (2) developing new interaction techniques, and (3) experimentally evaluating those techniques. Phase 1 used a controlled laboratory study with three adult age groups — young (18-54), pre-old (55-69), and old (70-85) — performing two pen-based selection tasks: multi-dimensional discrete tapping and menu selection. The study uncovered three primary sources of target acquisition difficulty: "missing just below" (tap distributions are systematically shifted downward, with a selection along the top edge of an item being 11 times more likely intended for the item above); "drifting" (users accidentally hover into adjacent menu regions when lifting their hand to see occluded content, causing menus to switch — affecting 35 of 36 participants and nearly doubling trial time); and "slipping" (landing on the correct target but sliding off before lifting, specific to older users and accounting for roughly half their errors).
Key findings
Phase 2 developed and evaluated two solutions for missing just below: a "reassigned edge" approach that shifts each menu item's target region down by 10% so taps along the top edge select the item above, and a "deactivated edge" approach that ignores taps on the top 10% of each item, creating an invisible separator. A follow-up experiment with younger (19-30) and older (66-81) adults revealed that participants fell into two distinct tap distribution groups — low hitters (downwardly shifted) and high hitters (upwardly shifted) — rather than showing a uniform downward shift. Only the deactivated edge approach benefited participants overall, though low hitters benefited from both approaches while high hitters were negatively impacted by the reassigned edge. For drifting, the simplest proposed solution is disabling hover-to-switch and requiring an explicit tap to change menus, with a possible delay-based alternative. For slipping, the proposed solution adapts Steady Clicks (a mouse cursor-freezing technique) to pen interaction, either by freezing internally without visual feedback, or by combining freezing with an area cursor that provides visual feedback and requires less positioning precision. The research demonstrates that individual user variability means no single solution works for all users, and technique choice depends on user characteristics.
Relevance
This research is highly relevant to touchscreen accessibility, which has become even more important since 2007 as smartphones and tablets have largely replaced PDAs and Tablet PCs. The three error types identified — missing just below, drifting, and slipping — remain common difficulties on modern touchscreen devices, particularly for older adults and people with motor impairments. The finding that tap distributions are systematically shifted downward has direct design implications: touch targets should account for this bias rather than centering the active region on the visual element. The observation that older users's slower, more overt interactions actually made it easier to identify difficulties that also affected younger users (who recovered too quickly to notice) is a powerful argument for inclusive design research — studying users with greater difficulty can reveal universal usability issues. The individual variability between low and high hitters underscores that accessibility solutions cannot be one-size-fits-all and may need to adapt to individual interaction patterns, anticipating the personalized accessibility settings now common in mobile operating systems.
Tags: aging · motor accessibility · input device · interaction design · target acquisition · mobile accessibility · touchscreen text entry