The Congressional Budget Office has released CBO’s Methods for Projecting Capital Gains Realizations:

The attached table shows capital gains tax rates, realizations, tax liabilities, and tax receipts since 1990….On the basis of CBO’s extrapolation of the available data for 2004, it appears that realizations increased by about 80 percent from 2002 to 2004.

CBO has updated its latest models with available data through 2004. Those models, which incorporate changes in the tax rate, fall well short of explaining the surge in realizations that occurred in 2004. Roughly half of the growth in realizations between 2003 and 2004 remains unexplained. After examining the historical record, including that for 2004, we cannot conclude that the unexplained increase is attributable to the change in capital gains tax rates. Volatility in gains can stem from other factors, such as changes in asset values, investor decisions, or broader economic trends.

Stuart Levine has more here.