Acceptability
System acceptability is the goal that designers should be aiming for and can be achieved by meeting the social and practical acceptability objectives for the system.
Usefulness, constituting usability and utility, has been identified as a key objective to providing practical acceptability. The description of practical acceptability needs to be extended to include accessibility.
Consequently, existing usability engineering techniques need to be adapted to include sensitivity to the prevalence of impairments.
The gap between usability and accessibility
Ideally when designing for accessibility, and thus for inclusivity, the users identified should represent the observed spread of capabilities in the population. Hence, some users should have high functional capability (low impairment), others should have moderate functional capability (moderate impairment) and finally there should also be users with low functional capability (high impairment).
However, identifying and securing access to representative users can be expensive and time-consuming, so an increasingly popular approach to inclusive design is to design for the more extreme end-users. The principal weakness of this approach is that unless the designers have been briefed that these users are still only a representative sample, and do not span the entire spectrum of users, then they may be tempted to optimise for those individual users only. Thus they may propose solutions that are in danger of becoming sub-optimal for many other users.