http://trac.syr.edu/tracirs/newfindings/current/

"The FY 2007 audit rate for the nation's largest corporations has plunged
to its lowest level in the last 20 years, less than half what it was in FY
1988, according to IRS data analyzed by the Transactional Records Access
Clearinghouse (TRAC).

The historic collapse in audits for the corporations with $250 million or
more in assets was especially notable during the last two years when the
rate dropped from 43% in FY 2005, to 34% in FY 2006 and then to an
all-time low of 26% in FY 2007.

But along with the declining number of audits for the largest
corporations, the IRS data point to a second significant finding: the
thoroughness of these essential audits has been dropping. One example of
this broad problem can be seen by the fact that the typical amount of time
auditors spend on each of the large corporate audits is down by 20 percent
over the last five years
<...>

Resource Shift to Auditing Smaller Corporations

The dramatic slump in the audits of the large corporations has occurred at
the same time that the IRS has assured Congress and the public that audits
for all corporations have been increasing.

Analysis of the IRS's data shows that the explanation for this apparent
contradiction lies in the changing enforcement strategies adopted by the
agency. By ordering its revenue agents to concentrate on the smaller
corporations that normally take a lot less time to audit, the agency was
able to push up the overall number of corporate audits while the
examination of the larger entities was sharply declining."