Because the sources for the two tables are different, the tax relief calculated for each state will not necessarily equal the sum of tax relief for that state's individual counties. In each case, the source is the most accurate data available for that specific level of analysis. Not every household files a tax return, however. Other individuals do not file because they do not have an income to report. Finally, some people simply do not file for personal reasons.
Many people maintain a permanent residence in one county or state but live in another.
These people pay taxes in a different county or state than where they live. Again the Census and IRS will account for these people in different locations. Many people file their tax returns late. Thus, the data from the 1991 IRS files may not include certain families that would be included in the Census count.