GAO reviewed the process used by the Internal Revenue Service (IRS) to select individual tax returns for audit. GAO found that most tax returns are selected for audit by a computer or a person other than the examiners who will audit them, and procedures generally protect the taxpayer against abuse. At district offices, most returns are selected because they have good audit potential. About 70 percent of returns audited by district offices are selected by a two-stage system. Returns are first scored as to their audit potential by a computer using sophisticated mathematical formulas. The highest scored returns are then manually screened to determine if an audit is warranted, and, in most cases, what items of income and deductions should be examined. Examiners can sometimes request returns for audit without having to explain why they need them. Overpayers are less likely to have their returns audited than those who underpay. Not enough is known about why taxpayers do or do not comply with the tax laws.