objectively anything like long-term limitations. Rather, the argument is that technology designs that are useful to people with certain long-term limitations might also be useful to people in certain disabling situations. A technology design for a person with one arm also might be useful for a person carrying an infant. Using an ability-based lens helps one recognize such design opportunities.

Assistive technology focuses mainly on compensating for long-term limitations within a person, as in Figure 5, bottom right. Ability-based design considers a larger space of limitations that impair technology use.

Design Principles

By adopting ability-based design in numerous projects, we have formulated and refined seven design principles to guide our work (see Table 2). The first three are required of any ability-based design project and relate to the designer’s attitude and approach, or “stance.” The next two relate to adaptive or adaptable user interfaces, and the final two to sensing and modeling users and contexts. Taken together, they can help guide designers and developers creating ability-based systems.

Example Projects

Our development of ability-based design was and continues to be highly iterative and inductive, arising from research projects that both preceded and followed its initial formulation. Here, we highlight a number of projects to illustrate the possibilities for ability-based design:

SUPPLE. SUPPLE9–11 was an auto-
matic user-interface generator that
used decision-theoretic optimiza-
tion to help choose interface widgets
and layouts that were optimized for
a user’s preferences, visual abilities,
and motor abilities. For optimizing
motor performance, SUPPLE first pre-
sented the user with a series of basic
pointing, clicking, dragging, and list-
selection tasks. 10 It then built regres-
sion models capturing the relation-
ship between task parameters and user
performance, using these models to
guide the optimization process such
that the interface being generated was
predicted to be the fastest to operate by
the user. Each user thus received a cus-
tom user interface, optimized for that
user’s particular abilities.

In a quantitative study in 2008 involving people with motor impairments, 11 SUPPLE’s custom interfaces were 26% faster and 73% more accurate to use than the default interfaces provided by manufacturers of popular desktop software applications. SUPPLE thus helped close more than 60% of the performance gap between people with and people without motor impairments, making access more equitable. Qualitatively, it was apparent how SUPPLE was optimizing interfaces based on different abilities; for example, SUPPLE gave people with muscular dystrophy interfaces with small, densely packed targets able to support slow, short, deliberate movements. In contrast, SUPPLE gave people with cerebral palsy interfaces with large, spread-out targets divided among different tabs, compatible with fast but error-prone movements. SUPPLE had no declarative knowledge of either muscular dystrophy or cerebral palsy, generating its user interfaces solely from observed input performance.

The SUPPLE approach was used in subsequent projects. For example, in SPRWeb, 6 SUPPLE’s personalized optimization approach was used to recolor websites, adapting them to the individual color-vision abilities of users with color-vision deficiencies. SPRWeb also aided users in color-limiting or color-altering situations, including glare and low-light conditions.

SUPPLE exhibited the first six principles of ability-based design and was the original system that inspired many of the ideas now found throughout ability-based design.

Slide Rule. Slide Rule14 was a mo-
bile screen reader that made touch-
screens accessible to blind users by
leveraging multi-touch gestures and
audio feedback. It was an example
of making systems usable to people
with abilities different from what de-
vice manufacturers originally intend-
ed. Slide Rule addressed a pressing
challenge emerging in 2007 from the
advent of touchscreen smartphones:
How would a blind person interact
with a phone having buttons that
person could not feel? At the time,
smartphones had little or no acces-
experience for individual users accord-
ing to their abilities and contexts.

Contexts Limiting Technology Use Ability-based design considers a broad range of contexts that impair technology use. We define a space with two axes: location and duration (see Figure 5). The location of a limitation ranges “from within the self” to “from outside the self.” Limitations arising from within the self are present in almost any context. Examples are a spinal cord injury, a toddler’s undeveloped psychomotor control, and being asleep. Changing a person’s context has little effect on the limitations arising from such internal states.

In contrast, limitations arising from outside the self are present primarily due to context, and therefore changeable. Astronauts have remarkable physical abilities, but while spacewalking, expressing many of those abilities is quite difficult. Even an Olympic athlete can do little when confined to a prisoner’s straightjacket. The external context severely limits the person’s expressible abilities.

Intermediate points also exist on the location axis, where the mixture of self and environment limit ability. One example of a mixed-location limitation is photosensitive epilepsy, where a flashing light might induce seizures. If not for the flashing light, seizures would not be triggered. In this example, a part of the person and a part of the environment combine to pose a possible limitation.

On the other axis, the duration of a limitation ranges from “ephemeral” to “enduring.” An ephemeral limitation lasts only briefly and changes quickly; one example is the lack of a usable arm because a person is carrying an infant. Next, short-term limitations can arise from many causes, including inebriation, illness, and an ankle sprain. Limitations might even be enduring or even lifelong, as with, say, those caused by age-related declines, spinal cord injuries, incurable diseases, lifetime imprisonment, or irreversible brain damage.

Our argument is not that the lived experience of a person with one arm is the same as that of a person carrying an infant. Situational impairments are neither subjectively nor