The relationship between accessibility and usability of websites

Accessibility and usability are well established concepts for user interfaces and websites. Usability is precisely defined, but there are different approaches to accessibility. In addition, different possible relationships could exist between problems encountered by disabled and non-disabled users, yet little empirical data have been gathered on this question. Guidelines for accessibility and usability of websites provide ratings of the importance of problems for users, yet little empirical data have been gathered to validate these ratings. A study investigated the accessibility of two websites with 6 disabled (blind) and 6 non-disabled (sighted) people. Problems encountered by the two groups comprised two intersecting sets, with approximately 15% overlap. For one of the two websites, blind people rated problems significantly more severely than sighted people. There was high agreement between participants as to the severity of problems, and agreement between participants and researchers. However, there was no significant agreement between either participants or researchers and the importance/priority ratings provided by accessibility and usability guidelines. Practical and theoretical implications of these results are discussed.