How big is Germany's corporate tax gap?

From Stefan Bach at the Deutsches Institut für Wirtschaftsforschung (DIW) in Berlin, a report entitled Has German Business Income Taxation Raised Too Little Revenue Over the Last Decades?

It's got plenty of detail in here, but we'll just highlight the top highlights:

"Our estimations suggest that the average implicit tax rate on German corporate income was around 21 percent since 2001. A detailed comparison of the corporate income measured in national accounts with the corporate tax base reported in the tax statistics reveals a considerable gap that amounts to 4 percent of GDP and more in 2004.
. . .
the gap still amounts to Euro 90 billion or 3.7 percent of GDP."

The full report is here.