IRS computers routinely make adjustments to correct mechanical errors in returns. In addition, the IRS conducts an extensive document matching computer program that compares taxpayer amounts of wages, interest, dividends, and other items to amounts reported by taxpayers. These programs automatically issue 30-day letters advising of proposed changes. Only a very small percentage of tax returns are actually examined. These are selected by a combination of computer analysis of return information and random sampling. The IRS has long maintained a program to identify patterns on returns most likely to require adjustment.