A software algorithm was utilized to define a preliminary set of PSUs satisfying the constraints. The input set consisted of all of the nonmetropolitan counties plus the seven MSA counties that were too small to stand as MSA PSUs (see PSU generation: Metropolitan Statistical Area Primary Sampling Units). The software formed PSUs that satisfied the minimum size constraints, respecting state boundaries (i.e., not crossing state boundaries). The software minimizes the maximum point-to-point distance for the candidate PSUs, while still satisfying the minimum size constraints (15,000 youths in the Northeast and Southeast NAEP regions, 10,000 youths in the Central and West NAEP 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, until all counties were assigned. In many instances there were counties that could not be combined into PSUs that satisfied the minimum size constraints while still remaining within a single state. In some cases, counties were combined across state lines to provide a PSU that met the minimum size requirement. In some cases, PSUs that were below the minimum size requirement were allowed to stand, if satisfying the minimum size requirement was not reasonably possible. In some cases, 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). 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 NAEP Region. The estimated number of youths (persons age 0 to 17) for each county comes from the 2000 Census Summary File 1 (SF-1) file.
|NAEP region||PSUs||Counties||Youths||Mean youths per PSU|
|NAEP region Northeast||60||107||1,468,000||24,500|
|NAEP region Southeast||223||752||4,826,000||21,600|
|NAEP region Central||281||840||4,285,000||15,300|
|NAEP region West||202||609||3,448,000||17,100|
|SOURCE: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, National Assessment of Educational Progress (NAEP), 2004.|