The Sample Frame

The first step in designing a sample is to define the population and the sample frame. In the case of people with developmental disabilities, the difficulty is to construct a list of sampling units—a frame. For example, the sampling frame for the NCVS is all U.S. households from which a nationally representative sample of 50,000 U.S. households comprising nearly 100,000 people is drawn. From this frame large segments of the general population, such as men, women, the elderly, and some minority groups such as blacks and Hispanics, can be identified, and the frequency characteristics and consequences of their victimization experiences can be explored through interviews. In the case of people with developmental disabilities, however, there is no exhaustive list that could serve as a sample frame. Household lists (of addresses, phone numbers, etc.) are problematic because a large proportion of this population does not reside in household units but in institutions, intermediate care facilities, and the like, so household lists would miss them. While this difficulty might be overcome, locating the population that resides in households also would require that the household member who answers the telephone or door identify a resident with a developmental disability as such. Many individuals with developmental disabilities will not admit having a disability, and some individuals without developmental disabilities will say they have a disability, causing incorrect households to be identified (see section below on screening bias).

The problems of using existing nonhousehold lists of persons with developmental disabilities are illustrated by the Client Development Evaluation Reports (CDERs) in California. In that state, an annual evaluation of each person with a developmental disability results in a CDER report. With some difficulty, these reports can be linked over time for tracking or monitoring individuals, or they can be arrayed in a cross-section for mea-