Tax Court Success Rate
Besides these, I keep track of my success rate in Tax Court. These are petitions filed in response to notices of deficiency.
Average reduction per case (dollar amount): $69,410
Average reduction per case (percentage): 77%
Cases which saw some reduction (percentage): 96%
This table of course only represents one type of dispute, and it doesn't include cases that were resolved with the IRS without the need to file a petition. (which is most of them) It also doesn't include cases where clients withdrew or otherwise canceled services. Nonetheless, it gives a general idea of what to expect.