Information on disability is an important component of health information, as it shows how well an individual is able to function in general areas of life. Along with traditional indicators of a population's health status, such as mortality and morbidity rates, disability has become important in measuring disease burden, in evaluating the effectiveness of health interventions and in planning health policy. Defining and measuring disability, however, has been challenging. The World Health Organization (WHO) has tried to address the problem by establishing an international classification scheme known as the International Classification of Functioning, Disability and Health (ICF). (1) Nevertheless, all standard instruments for measuring disability and health need to be linked conceptually and operationally to the ICF to allow comparisons across different cultures and populations.
To address this need for a standardized cross-cultural measurement of health status and in response to calls for improving the scope and cultural adaptability of the original World Health Organization Disability Assessment Schedule (WHODAS), (2-9) WHO developed a second version (WHODAS 2.0) as a general measure of functioning and disability in major life domains. This paper reports on the development strategy and the metric properties of the WHODAS 2.0.
Conceptual framework for WHODAS 2.0
The WHODAS 2.0 is grounded in the conceptual framework of the ICF and captures an individual's level of functioning in six major life domains: (i) cognition (understanding and communication); (ii) mobility (ability to move and get around); (iii) self-care (ability to attend to personal hygiene, dressing and eating, and to live alone); (iv) getting along (ability to interact with other people); (v) life activities (ability to carry out responsibilities at home, work and school); (vi) participation in society (ability to engage in community, civil and recreational activities). All domains were developed from a comprehensive set of ICF items and made to correspond directly with ICF's "activity and participation" dimension (Table 1), which is applicable to any health condition. For all six domains, the WHODAS 2.0 provides a profile and a summary measure of functioning and disability that is reliable and applicable across cultures in adult populations.
The WHODAS 2.0 is used for many purposes. It can be used for conducting population surveys, (10-15) for registers (16) and for monitoring individual patient outcomes in clinical practice and in clinical trials of treatment effects. (17-27)
The WHODAS 2.0 was constructed through a process involving extensive review and field-testing, as described in the following sections.
In preparation for the development of the WHODAS 2.0, we conducted a review of existing measurement instruments and of the literature on the conceptual aspects and measurement of functioning and disability. The instruments we chose included various measures of disability, handicap, quality of life and other aspects of health, such as the ability to perform the activities of daily living (including instrumental ones), as well as global and specific measures of well-being (including subjective well-being). (28,29) We compiled information from more than 300 instruments in a database showing a common pool of items, along with the origin and known psychometric properties of each instrument. An Instrument Development Task Force composed of international experts reviewed the database and pooled the items in it using the ICF as the common framework.
Research study and field testing
Since the WHODAS 2.0 was developed primarily to allow cross-cultural comparisons, it was based on an extensive cross-cultural study spanning 19 countries around the world. (30) The items included in the WHODAS 2.0 were selected after exploring how health status is assessed in different cultures through a process that involved linguistic analysis of health-related terms, interviews with key informants and focus group discussions, as well as qualitative methods (e. …