The Medical Model of Disability
Disabilities have traditionally been described with reference to medical conditions that they were seen to arise from. This is known as the medical model of disability and was encapsulated in the 1980 World Health Organisation’s (WHO) International classification of impairments, disabilities, and handicaps  which included the following definitions:
- Impairment = a loss or abnormality of physical bodily structure or function, of logic-psychic origin, or physiological or anatomical origin
- Disability = any limitation or function loss deriving from impairment that prevents the performance of an activity in the time-lapse considered normal for a human being
- Handicap = the disadvantaged condition deriving from impairment or disability limiting a person performing a role considered normal in respect of their age, sex and social and cultural factors
The Social Model of Disability
The main alternative to the medical model of disability is the social model. This has been highly influential, over the last 30 years, in shaping policy, practice and attitudes to disabled people. The social model stemmed from the publication of Fundamental Principles of Disability in 1976.  This revolutionised the understanding of disability arguing that it was not mainly caused by impairments but by the way society was organised and responded to disabled people.
In the social model, disability is caused by society and is not the ‘fault’ of an individual disabled person, or an inevitable consequence of their limitations. Disability is the product of the physical, organisational and attitudinal barriers present within society. The social model takes account of disabled people as part of the economic, environmental and cultural society.
The WHO revised its definitions of disability, in part as a response to this social model, and from the realisation that the medical model was of very limited use in defining effective responses in meeting the needs of disabled people. In 2001 WHO published the International Classification of Functioning, Disability and Health (ICF) . In the ICF disability is described as:
… the outcome or result of a complex relationship between an individual’s health condition and personal factors, and of the external factors that represent the circumstances in which the individual lives.
Building on the social model the IMS Global Learning Consortium, introducing its work developing technical standards for accessibility in e-learning, offered a more education specific definition of both disability and accessibility :
… the term disability has been re-defined as a mismatch between the needs of the learner and the education offered. It is therefore not a personal trait but an artifact of the relationship between the learner and the learning environment or education delivery. Accessibility, given this re-definition, is the ability of the learning environment to adjust to the needs of all learners. Accessibility is determined by the flexibility of the education environment (with respect to presentation, control methods, access modality, and learner supports) and the availability of adequate alternative-but-equivalent content and activities. The needs and preferences of a user may arise from the context or environment the user is in, the tools available (e.g., mobile devices, assistive technologies such as Braille devices, voice recognition systems, or alternative keyboards, etc.), their background, or a disability in the traditional sense. Accessible systems adjust the user interface of the learning environment, locate needed resources and adjust the properties of the resources to match the needs and preferences of the user.
Post Social Models of Disability
The social model of disability has been criticised and various moves instigated to move beyond it. For an example see Torn Shakespeare and Nicholas Watson (2001) . They argue instead for an Embodied Ontology: “we are our body, with all of its imperfections and impairments”. Further, they assert that “there is no qualitative difference between disabled and non-disabled people because we are all impaired in some form, some more than others”. They consider the idea of a normal/perfect person as mythical. However, this discussions has been more within the academic world of disability studies and I would contest has yet to have widespread impact beyond this, and particularly relevant to this post, on accessibility. That being said one direct relation to accessibility is that accessibility accommodations have benefits for many who do not consider themselves disabled. An example of this is the feature present in most modern browsers to enlarge the display of web pages in response to a short cut key, usually Ctrl +. This was originally introduced for those with a visual impairment but at times is useful to all. A major piece of research undertaken by Forrester for Microsoft in 2003  supports this case of the wider benefit of accessibility accommodations. It found that 57% of working-age computer users are likely to benefit from accessible technology (where accessible technology is understood as technical responses to promote access for disabled people to computer hardware and software).
Functional Models of Disability
The term accessibility is widely used in the context of web design. The W3C describes web accessibility thus:
Web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the Web, and that they can contribute to the Web. 
This is in essence based on a functional model of disability. Generally in Human Computer Interaction (HCI) a functional approach is most useful. What is important in the design of web-based applications or content is how the diversity of users access the computer. This design can be said to be accessible if it facilitates full interaction by all users irrespective of assistive technologies or access approaches that may be adopted by some.
The AccessForAll 3.0 Personal Needs and Preferences (PNP) provides a specification that enables comprehensive profiles of individuals’ access approaches and assistive technologies to be stored based on a functional model. This specification is being developed within the IMS Global Learning Consortium and went to public draft in September 2012 . These functional profiles could be generated by disabled people themselves, possibly with the help of advisors, inputting their specific access approaches and requirements to a web-form. Such profiles have great potential in personalisation approaches to accessibility and in analytics based approaches to identifying accessibility issues, as discussed elsewhere in this blog.
A Note on WCAG and Models of Disability
The Web Content Accessibility Guidelines (WCAG) 2.0 , a formal recommendation of the web standards body the W3C, are the de facto international standard on web accessibility. These are targeted at web developers and cover what is normally referred to as technical accessibility. They are organised according to four top-level principles of web accessibility: that web pages should be perceivable, operable, understandable, and robust. WCAG are focussed at the properties of a web page and in so doing might be considered to be based on a functional model. However, the user is deliberately subsumed in their formulation; their concern is the functional properties of the web page not the person accessing them. This ignores the consequences of the social model of disability of the importance of context and the relational nature of accessibility.
The development of web assets or applications is a process. Accessibility considerations need to be built into the everyday practices across the web product life-cycle from conception and specification through development to delivery and maintenance. Recognising this, the British Standards Institute developed BS 8878: 2010 Web Accessibility Code of Practice . This facilitates a pragmatic application of WCAG 2.0 within a process based approach and reasserts a user focus.
Our models of disability are important, they shape our attitudes and impact on how effectively the needs and preferences of disabled people are met in design. The medical model is now widely seen as outmoded and a perpetuator of discriminatory attitudes. The social model has had widespread influence. It is important in accessibility considerations because it recognises the importance of the context of the users and supports the view of accessibility as a relationship property; in the case of web accessibility the relationship being between the diversity of users and the web resource or application. Functional models have been asserted as the most useful in design and development and the potential of these for personalisation and analytics highlighted.
(All web-links checked 10 October 2012)
 World Health Organization, (1980) International classification of impairments, disabilities, and handicaps. A manual of classification relating to the consequences of disease. Geneva, WHO
 UPIAS, (1976) Fundamental Principles of Disability, London: Union of Physically Impaired against Segregation, available on-line at:
 World Health Organization. (2001) International Classification of Functioning, Disability and Health. Geneva, WHO, Searchable online versions available at: http://www.who.int/classifications/icf/en/
 IMS Global Learning Consortium (2004) IMS AccessForAll Meta-data Overview. http://www.imsglobal.org/accessibility/accmdv1p0/imsaccmd_oviewv1p0.html
 Torn Shakespeare, Nicholas Watson, (2001) The social model of disability: An outdated ideology?, in Sharon N. Barnartt and Barbara M. Altman (ed.)Exploring Theories and Expanding Methodologies: Where we are and where we need to go (Research in Social Science and Disability, Volume 2), Emerald Group Publishing Limited, pp.9-28. Availible on-line at:
 Microsoft (2004) The wide range of abilities and its impact on computer technology. Available on-line at:
 World Wide Web Consortium (2005). Introduction to Web Accessibility, available at: http://www.w3.org/WAI/intro/accessibility.php
 IMS Global Learning Consortium (2012) Access for All (AfA), Version 3.0 Specification, Public Draft 1.0. Primer and specification documents available from: http://www.imsglobal.org/accessibility/index.html
 W3C (2008), Web Content Accessibility Guidelines 2.0 (WCAG 2.0), available at: http://www.w3.org/TR/WCAG/
 British Standards International (2010). BS 8878:2010 Web Accessibility – Code of Practice, (charged for publication available through http://www.bsi-publications.com and by subscription through BSOL https://bsol.bsigroup.com/)