How to Measure Disability and Why It Is Important

In this article, Professor Hannah Kuper (Lead Educator, LSHTM), Dr. Sarah Polack (LSHTM) and Dr. Islay Mactaggart (LSHTM) discuss why and how to measure disability across different populations.

The importance of measuring disability

The World Report on Disability estimates that there are about one billion people with disabilities in the world.¹

However, as we will learn later this week, there are gaps in the global data we have on disability, and so this figure is rather uncertain.

There are a number of important reasons why we should collect data on disability:

• For advocacy, so that we can promote the full inclusion of people with disabilities in their societies on an equal basis with others.
• For programme planning, so that we know how many people with disabilities there are, and the contexts that they live in. We need to know this to plan programmes that adequately meet their needs.
• Because the Sustainable Development Goals (SDGs) describe the need for inclusive development that “Leaves no one behind”. We must ensure that data on disability is included in all reporting on achievements towards the SDGs so that we can assess how far each goal is achieved for people with disabilities and ensure they are not left behind.

Approaches to measuring disability

There are three main approaches to measuring disability which are compatible with the international classification of functioning, disability and health (ICF) model:

Direct questioning on disability

The first approach to measuring disability is to directly ask people whether they view themselves as being disabled or having a disability. For example, the Zambia census in 1990 asked each person “Do you have a disability? Yes/No”. This approach is simple and quick, but is likely to severely underestimate the prevalence of disability as people may not consider themselves to be disabled, or fear stigma or discrimination if they are labelled as disabled. It is therefore recommended that direct questioning is not used to measure disability.

Self-reported functioning

A second approach is to measure disability through self-reported functioning; that is, asking people whether they experience difficulties in different functional domains.

The Washington Group on Disability Statistics ², a United Nations City Group, have developed a short set of questions which aims to capture the proportion of the population living with different levels of functional limitation. These questions ask whether a person experiences difficulties in six basic functional domains: seeing, hearing, walking, cognition, communicating and self-care.

The Washington Group Short Set of Questions:

1. Do you have difficulty seeing, even if wearing glasses?
2. Do you have difficulty hearing, even if using a hearing aid?
3. Do you have difficulty walking or climbing steps?
4. Do you have difficulty remembering or concentrating?
5. Do you have difficulty (with self-care such as) washing all over or dressing?
6. Using your usual (customary) language, do you have difficulty communicating, for example understanding or being understood?

Response options for all questions: No, no difficulty; yes, some difficulty; yes, a lot of difficulty; cannot do at all

These questions are widely recommended for data collection on disability; they are simple, quick, reproducible and easy to translate into different languages. They are suited for use in censuses or in large surveys where only a few questions can be included, and as they are used widely, the results can be compared over time and between countries. They are non-stigmatizing, as they do not ask about disability directly.

However, the Short Set provides limited data on other key components of disability, such as participation restrictions or impairments. They also exclude certain important functional domains such as those related to psychosocial functioning and mental health.

The Extended Set of Washington Group questions contains up to thirty five questions, and can be used in surveys where more time is available. They capture a more complete picture of disability. Additional domains include psychosocial functioning, pain, fatigue and upper body function, as well as more in-depth questions related to the basic domains of the Short Set (e.g. separately asking about near and distance vision). The Washington Group have also collaborated with UNICEF to develop an extended set of questions on functioning for children aged 2 to 17.

The Model Disability Survey (MDS)³ provides another self-reported approach to measuring disability. The MDS is a general population survey tool that provides detailed information on the lives of people with disabilities. It views disability as the outcome of interactions between a person with a health condition and various environmental factors.

The MDS considers that disability is a continuum that ranges from low to high levels of severity and measures both what the person is capable of doing in their current environment (performance) and what they would be capable of doing in an fully inclusive and accessible environment (capacity).

Assessment of impairments or health conditions

Impairments or health conditions are components of disability that can be measured directly using objective testing. The Rapid Assessment of Avoidable Blindness (RAAB)⁴ for example, is designed to objectively measure visual impairment in the population. Impairment and health condition data are important for planning appropriate health and rehabilitative services amongst those who would benefit from these (e.g. provision of cataract surgery, hearing aids, mobility devices). This may be particularly needed in low resource settings where inadequate access to health care is closely related to disability.

However, it is important to recognise that impairment and health condition testing in isolation generally does not consider how it affects the individual’s level of functioning or participation. This type of assessment may also be more resource intensive to undertake than gathering self-reported information. However, recent advances in technology are increasing the ability of non-clinical interviewers to undertake short impairment tests alongside self-reported functioning tools. One example is Peek Acuity, which allows the assessment of visual acuity and other eye measures by a non-specialist using a smartphone.⁵

Visual Acuity testing during a survey in Mexico. Photo Credit: CC-BY-NC 2.0/ Sarah Polack ICEH

Evidence from the field

A survey by ICED of around 8000 people in India and Cameroon compared these different approaches for measuring disability.^6.7 The survey used the Washington Group Questions, and participants were also assessed by clinicians for the presence of various impairments. In addition, in India people were asked whether they considered themselves to have a disability.

Overall findings were that:

• Using the Washington Group Extended Set was straightforward and took on average 10-15 minutes per person, while the Short Set questions took 3-5 minutes. Prevalence of disability using the Short set was 7.5% in India, and 5.9% in Cameroon.

• Measurement of impairments was more difficult and expensive, and depended on the presence of clinical staff, but did provide useful information (e.g. the population need for hearing aids). Prevalence of any clinical impairment was 10.5% in India, and 8.4% in Cameroon.

• The combined prevalence of disability (Washington Group questions and clinical impairments) was 12.2% in India and 10.5% in Cameroon, showing the added benefit of collecting both impairment and self-reported data.

• In India, 3.8% of people reported that they had a disability on the single question, compared to 12.2% who were found to have any disability. This demonstrates that the single question severely underestimates the prevalence of disability.

Conclusion

There is a growing emphasis on measuring disability in order to 1) estimate the prevalence of disability, 2) identify the needs of people with disabilities, and 3) monitor the inclusion of people with disabilities.

There are different approaches to the measurement of disability, which have advantages and disadvantages (Table 1, above). A combined approach using both self-report and assessment of impairments may be beneficial.