persons with disabilities vary as a function of differences in the specific wording of the question, the number of questions used to determine the prevalence and severity of impairments and disabilities, the context of the questions immediately proximate to the question of interest, and the overall focus of the questionnaire (health versus employment versus program participation).

EMPIRICAL EVIDENCE CONCERNING MEASUREMENT OF DISABILITY ERROR

To date, most investigations with respect to the error properties associated with the measurement of persons with disabilities or the measurement of persons with work disabilities have focused on errors of observation, ignoring differences in estimates due to coverage error and nonresponse error. This review of the empirical literature is therefore focused on errors of observation. As an illustration of the type of empirical investigations concerning error in the measurement of disability, this section begins by examining the work that has been done to date with respect to measures of activities of daily living (ADL). The intent is to provide an illustration of the type of work that has been done (and not done) with respect to a frequently used measure of functional limitation. The focus is then turned to the measurement of persons with work disabilities.

Measurement of ADLs, Functional Limitations, and Sensory Impairments

Although there are several different measurement methods for the assessment of physical disability, one of the most often used (within the context of survey measurement) is the Index of Activities of Daily Living, often referred to as the Index of ADL (Katz et al., 1963). The index was originally developed to measure the physical functioning of elderly and chronically ill patients, but several national surveys of the general population administer the index to adults of all ages. The index assesses independence in six activities: bathing, dressing, toileting, transferring from a bed or chair, continence, and feeding. Despite its wide acceptance and use, the psychometric properties of the index have not been well documented. Brorsson and Asberg (1984) reported reliability scores of 0.74 to 0.88 (based on 100 patients). Katz et al. (1970) applied the Index of ADLs as well as other indexes to a sample of patients discharged from hospitals for the chronically ill and reported correlations between the index and a mobility scale and between the index and a confinement measure of 0.50 and 0.39, respectively. Most assessments of the Index of ADLs have examined the predictive validity of the index with respect to independent liv-



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