They let the cat out of the bag. I did some poking around the 2006 Budget and found the following on p. 191. I have posed the question here and there: How do the Presidents' men predict productivity when they are talking tax cuts? The answer is here. "conservatively, to be 2.6% per year". How then do they get away with 2.1% as their optimistic number when talking Trust Funds?
2006 Budget: Analytical Perspectives
"Potential growth is approximately equal to the sum
of the trend rates of growth of the labor force and
of productivity. Potential GDP growth is projected to
be 3.2 percent through 2008, and then edge down to
3.1 percent during 2009–2010, primarily because of an
assumed slowing in labor force growth. The labor force
is projected to grow about 1.2 percent per year through
2008 on average, slowing to about 0.8 percent yearly
on average during 2009–2010 as increasing numbers
of baby boomers enter retirement.
Trend productivity growth is assumed, conservatively,
to be 2.6 percent per year. That pace is noticeably below
the average since the business cycle peak in the first
quarter of 2001 (4.2 percent per year). It is, however,
close to the pace during 1996–2000 (2.5 percent) and
not far from the average since the official productivity
series began in 1947 (2.3 percent)."