To our knowledge, no country actually does a calculation of the international tax gap. There are some that have attempted a measurement using a lot of assumptions of a domestic tax gap, which is sometimes thought of as the underground economy, but not on the international tax front.
I understand that this is, or is intended to be, a continuation of hearings that were held a couple of years ago. During that set of hearings, you heard from Mr. Jeffrey Owens, the head of the tax section at the OECD. He said then—and I'm paraphrasing a little bit—that trying to measure the tax gap is prone to conceptual difficulties and leads to practical difficulties. If you could measure it accurately, you could probably tax it quite simply.
That's probably a fair assessment of the issue.