The report is: Tax Gap: IRS Could Significantly Increase Revenues by Better Targeting Enforcement Resources, GAO-13-151 (dated December 5, 2012)
Read the GAO report: GAO-13-151 [PDF 1 MB]
Summary of GAO findings
The GAO estimated that the average cost (including overhead) of correspondence exams opened in 2007 and 2008 was $274, compared to an average of $2,278 for field exams.
The GAO found that the IRS spent almost 20% of the $1.6 billion per year that it devoted to exams on returns from taxpayers with positive income of at least $200,000, even though such returns accounted for only 3% of the 136 million individual returns filed per year.
The GAO estimated that, for the two years of cases reviewed, correspondence exams were significantly more productive in terms of direct revenue produced per dollar of cost than field exams. Both types of exams of taxpayers with positive incomes of at least $200,000 were significantly more productive than exams of lower- income taxpayers.
The GAO demonstrated how these estimates could be used to inform resource allocation decisions. For example, a hypothetical shift of a small share of resources (about $124 million) from exams of tax returns in less productive groups to exams in the more productive groups could have increased direct revenue by $1 billion over the $5.5 billion per year that the IRS actually collected (as long as the average ratio of direct revenue-to-cost for each category of returns did not change). These gains would recur annually, relative to the revenue that IRS would collect if it did not change its resource allocation. This particular resource shift would not reduce exam coverage rates significantly and, therefore, the GAO found this would have little, if any, negative effect on voluntary compliance.