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These are some questions to ask and points to keep in mind when using survey data.
Each survey has a different purpose, which determines the way each survey collects data and how it treats the concept of disability. For example, the National Health Interview Survey (NHIS), conducted by the National Center for Health Statistics, is done to monitor the health of the U.S. population. The current version of the NHIS defines disability in terms of limitations in a person's activities due to a health condition or impairment. "Activities" according to the NHIS is broad enough to include working, doing housework, taking care of personal and household needs, and other age-appropriate activities. In contrast, the Current Population Survey. meant to provide labor market data for the Bureau of Labor Statistics, addresses disability narrowly in terms of work limitation.
A survey that provides useful national-level data may not offer reliable data at the state or local level. For example, the U.S. Census yields data on geographic areas as small as individual blocks. The National Health Interview Survey, because of its small sample size, can not provide reliable state- or local-level estimates.
Because of the variation in the purposes of the major surveys and the way they define "disability" (as well as other variables), data from one survey may not be directly comparable with data from another.
Surveys often undergo revision from one year to the next. When you notice a change in a quantity, such as the disability rate, be sure that neither the questionnaire nor the survey procedures have been substantially modified during that period.
Nearly all Web sites for the major federal surveys that collect disability data provide content descriptions for each new version of the survey, as well as copies of the questionnaires and, in some cases, a glossary that describes how each variable is defined. In order to correctly interpret data from the survey, it is wise to study the documentation in order to understand the exact questions that were asked, the way variables were defined, the population that was sampled, and the level of statistical reliability of the statistics you obtain.
Please read How to find disability data on the Web to learn more about the purpose, sampling strategy, limitations of the data, and other aspects of the major nationally representative surveys.