1. 3. US Department of the Census . Data published electronically at Internet address http://www.census.gov/population/esti-mates/nation/intfile2-1.txt (accessed1999 ).
2. 4. US Department of the Census . Data published electronically at Internet address http://www.census.gov/population/projec-tions/nation/nas/npas0610.txt (accessed1999 ).
3. 5. US Department of the Census . Data published electronically at Internet address http://www.census.gov/population/projections/nation/nas/npas1530. txt (accessed1999 ).