A software algorithm was utilized to define a preliminary set of primary sampling units (PSUs) satisfying the constraints. The input set consisted of all of the non-metropolitan counties. The software formed PSUs that satisfied the minimum size constraints, respecting state boundaries (i.e., not crossing state boundaries). The software also minimized the maximum point-to-point distance for the candidate PSUs, while still satisfying the minimum size constraints (15,000 youths in the Northeast and South Census regions, and 10,000 youths in the Midwest and West Census regions). "Worst first" was the general approach: the county that had the PSU with the largest maximum point-to-point distance was fitted first, with those counties that best fit a PSU containing the "worst-first" county. The algorithm was then run on the remaining counties not yet assigned to a PSU.
Initially, there were 22 counties that could not be combined into PSUs that satisfied the minimum size constraints while still remaining within a single state. Nine of the PSUs (formed from 12 of these counties) that were below the minimum size requirement were allowed to stand, since satisfying the minimum size requirement was not reasonably possible. The remaining counties were in Alaska, for which PSUs were manually drawn to better respect interstate highways (being drawn along the axis of these highways) and mountain ranges (avoiding crossing of ranges with poor road access). For Alaska, the proto-PSUs created by the program were replaced by the PSUs created for the NAEP 2004 Assessment. The end result of this procedure was that all non-metropolitan PSUs were contained within state boundaries. There were a total of 670 final non-metropolitan PSUs.
The table below presents the number of PSUs, the number of counties represented, and the estimated number of youths (total and mean per PSU) by Census region. The estimated number of youths (persons age 0 to 17) for each county comes from the 2003 U.S. Census Bureau's Population Estimated Program.
|Census region||PSUs||Counties||Youths||Mean youths per PSU|
|SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2006.|