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The United States is divided into 107 primary sampling units, which are further divided into clusters by postal ZIP codes. The Local Educational Authorities and state-supported special schools were the primary sampling unit from which a A 2-stage cluster design, probability proportional to cluster size, was used to generate a probability sample of 200 households within the study area; 20 primary sampling units were selected at the first stage and 10 households at the second stage (Figure 1). |
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