Chapter 2 Background
2.3 Understanding Users with Disabilities
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user would mean to fully understand the target population along all of the dimensions.
Therefore this categorisation highlights the complexity and wide range of consideration that must be given when designing inclusively.
Table 2-3 User Characteristics Relevant to Product Design
Domain Considerations
1. Sensory Capabilities Vision, hearing, taste, smell, touch, balance, kinaesthesia
Aids for vision and hearing
2. Cognitive Capabilities Information processing capacity and performance (perception, working memory, attention, long term memory)
High level cognition: reasoning, planning, problem solving
Knowledge (declarative and procedural), experience and mental models of products
3. Motor Capabilities Static (body dimensions) and functional anthropometry (range of movement)
Strength with various body postures
Fine motor skills (hand-eye coordination, dexterity)
Gross motor skills (body movement, locomotion)
Endurance and stamina
Aids for movement 4. Personal and cultural factors Preferences
Aspirations
Motivation
Language 5. Socio-economic factors Age
Gender
Income
Lifestyle
Education
Family support
6. Health Status/Disability Medical Conditions (short and long term effects)
Accident/injury (short and long term effects)
Medication effects
Pregnancy
2.3.1 Understanding Disability: The ICF Model
The concept of disability needs to be understood as a precursor to Inclusive Design, as many people with functional capability loss are said to have one or more ‘disabilities’. In the field of disability studies, there are complications in rigidly defining disability because “disability is a complicated, multidimensional concept” (Altman, 2001). The World Health Organisation (World Health Organisation, 2001) defines disability as “Any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being.” For further information on the development of disability definitions, classification schemes, and models up to the current version of the International Classification of Functioning, Disability and Health, the reader is referred to (Altman, 2001).
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The International Classification of Disability, Functioning and Health (ICF) (World Health Organisation, 2001) and the International Statistical Classification of Diseases and Related Health Problems (ICD-10) (World Health Organisation, 1992) together provide a
comprehensive language for disability and health problems. The ICF shows a move from a medical model of disability toward a social model of disability. The medical model suggests that disability is a trait of the individual, while the social model suggests that disability arises because of the mismatch between the individual and the social and physical environment.
This means that disability is caused by the demands of the environment rather than being a specific attribute of the person. Thus a person is disabled only when viewed in the context of the environment in which he or she must function.
The model of disability given by the World Health Organisation (WHO) is shown in Figure 2-5 (World Health Organisation, 2001). This model highlights the complexity and interaction between the concepts of medical and social models of disability. Various health conditions impact on a person’s body structures and functions, thus affecting the person’s activities and participation in the social environment. Environmental and personal factors also contribute to the person’s body functions, activities and participation, resulting in a complex model of inter-relating factors. The ICF model is now widely adopted and used as the basis for the collection of data on disability for medical and decision-making purposes. Since it recognises the complexity of disability as a transactional process between multiple factors, it is an appropriate model for understanding and framing disability issues.
Figure 2-5 The model proposed by the World Health Organisation for Disability in the International Classification of disability, functioning and health
From the ICF model, definitions for common terms are set out as shown in Table 2-4.
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Table 2-4 Definitions of Disability and related concepts (World Health Organisation, 2001)
Concept Definition
Disability Any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being.
Body Functions Body Functions are physiological functions of body systems (including psychological functions).
Body Structures Body Structures are anatomical parts of the body such as organs, limbs and their components.
Impairments Impairments are problems in body function or structure such as a significant deviation or loss.
Activity Activity is the execution of a task or action by an individual.
Participation Participation is involvement in a life situation.
Activity Limitations Activity Limitations are difficulties an individual may have in executing activities.
Participation Restrictions
Participation Restrictions are problems an individual may experience in involvement in life situations.
Environmental Factors Environmental Factors make up the physical, social and attitudinal environment in which people live and conduct their lives.
From these definitions, though a user may have impairments, disability occurs when a user cannot perform a required activity. Thus from a design perspective, if the product and environment is changed to accommodate the user’s capability, the apparent disability would be removed. This analysis leads to the term ‘disabling by design’, as it is within the designer’s power to create products and environments that accommodate people who may not have
‘average’ capability.
From an Inclusive Design perspective, the ICF model of disability demonstrates that disability is a continuum rather than a binary category. To consider a person as being disabled or not- disabled is a gross simplification of disability, ignoring the many levels and combinations of sensory, cognitive and motor functional loss that can occur. The ICF model also forms the basis of an extensive taxonomy of body functions, structures, activities and participation that is very comprehensive and medically oriented. A few of these aspects are relevant to product design, however in the most part, such classifications are unsuitable for general design purposes (Carlsson et al., 2002).
2.3.2 Disability Data
In reviewing disability definitions and statistics on the prevalence of disability in the UK population, a major issue was discovered. There are no “Gold Standard” definitions of disability due to the fact that disability is a multidimensional and dynamic concept (Bajekal, Harries, Breman, & Woodfield, 2004; Tibble, 2004). Because of this, the population estimates
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of disability from different surveys are not directly comparable, and users of these disability estimates are required to choose a survey estimate based on their particular needs (Bajekal et al., 2004). This leads to problems when trying to collate and compare disability prevalence data from different sources.
Fujiura presents an overview of Disability Measurement Systems in the Handbook of Disability (Fujiura & Rutkowski-Kmitta, 2001). Essentially, there are three sources of population-level disability data: national censuses, household surveys and administrative registries. Because samples for censuses and household surveys are aimed at statistical accuracy, the data collection is costly in terms of labour and resources. This in turn limits the depth and detail of disability measurement instruments, which means that the resulting data is limited to a few indicative measures that are relatively easy to collect. For the UK, Bajekal presents an overview of disability surveys available (Bajekal et al., 2004) and the context of their estimations based on the definition of disability used. The UK Department of Work and Pensions also gives a table of disability prevalence from different surveys (Tibble, 2004) which stands at 22% of the UK population using the Disability Discrimination Act (DDA) as the definition of Disability.
From a public health and medical standpoint, surveys with disability elements tend to measure the performance of individuals on tasks such as Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL) with the aims of improving health and quality of life (Bajekal et al., 2004). Though ADLs and IADLs include activities such as eating, bathing, shopping and manageing money, measurements with respect to specific product interface features are usually not available. As disability surveys provide data for policy-making purposes or for population health purposes, the field of Human Factors and Ergonomics should aim to characterise and measure the capability of disabled users on a population level, specifically to support Inclusive Design.
Another problem with population data is that it quickly becomes outdates due to changing demographics (Yoxall et al., 2006). For example, 95th percentile strength data for older adults collected in the past would not represent current 95th percentile strength data in the current context of rapid population change. The only way this can be remedied is via standardised measurement at regular intervals together with statistical estimation tools that would enable better estimates of true population data.
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2.3.2.1 Focusing on Functional Capacity for Design
Disability can be characterised by disease (etiology), onset, progressive nature (time varying) and severity (Sesto, Vanderheiden, & Radwin, 2004). Even though ageing, disease and trauma account for limitations in functional capacity, information on the causes of functional ability loss are less relevant than the actual levels of functional ability. It is more important to understand what a user can and cannot do, and what levels of difficulty are experienced, rather than focusing on the cause of the disability (medical model) (Cardoso, 2005; Sesto et al., 2004). However, at times it may be useful to understand the causes of disability such as medical conditions, trauma and medication effects in order to explore contextual and lifestyle factors of a particular user group (Persad et al., 2005).
Sesto also puts forward the concept of ‘functional equivalence’ where different medical conditions can cause similar problems in functional limitations, and common solutions could be found to satisfy these groups of conditions. Thus capability (or functional capacity) characterisations appear to be the most appropriate for product design and evaluation because information on what users can and cannot do could be directly translated into improvements in interface features (Carlsson et al., 2002; Jacko & Vitense, 2001; Sesto et al., 2004).
2.3.3 Section Summary
In this section, the definition of disability and the ICF model were reviewed. It was shown that population representative disability surveys are limited in the information that they could provide, and the concept of capability or functional capacity should be used to capture data on disabled populations. In the next section, product evaluation methods for Inclusive Design are reviewed.