The Fed initially disappointed some investors on Thursday when it said it would buy $40 billion of mortgage-backed securities each month. That is far less than the $75 billion a month it bought in its second round of bond-buying, or the more than $100 billion monthly tab for its first round.

　　The U.S. Federal Reserve's third round of bond-buying could ultimately rival the size of its first huge quantitative easing, which was widely seen as boosting growth.

　　But this time, the Fed has promised that "if the outlook for the labor market does not improve substantially," it won't stop buying and could ramp up its spending further.
　　Depending how the Fed defines "substantially" and how long it takes to get there, it could end up buying bonds for several years, adding $1.7 trillion or more to its balance sheet, analysts say.
　　By comparison, the Fed's initial round of quantitative easing, first announced in November 2008 as the U.S. economy slumped into a deep recession, totaled $1.75 trillion .
　　"They've clearly committed to do what it takes to get unemployment down where they want it," said Pierre Ellis, an economist at New York-based Decision Economics. "There's no limit."
　　To Ellis, the "big bazooka" of open-ended bond purchases -- designed to boost the economy by lowering borrowing costs -- won't have much immediate impact because the economy's main headwind is uncertainty over fiscal policy and the outcome of a presidential election.
　　That's a view shared by several of the central bank's hawkish members.
　　But many other analysts say the Fed's latest program may exceed its predecessors in size, and more importantly, also pack the desired punch.
　　"We believe that with a strong commitment from the Fed, progress in Europe and the passing of the U.S. election, the U.S. economy will have a pretty decent shot at achieving above-trend growth in 2013," Julia Coronado, an economist at BNP Paribas, wrote in a note to investors.
　　Thursday's announcement could add another $1.2 trillion to $1.7 trillion to the Fed's balance sheet, she said.
　　TIED TO THE MAST
　　The Fed has kept short-term rates near zero since December 2008, and Fed Chairman Ben Bernanke has led the U.S. central bank into ever-newer territory to lower real rates further.
　　After the first round of bond-buying in 2008 and 2009, the Fed resorted in 2010 and 2011 to a second round to ward off deflation as the recovery faltered.
　　As he doubles down on quantitative easing, Bernanke's approach looks flexible enough to win support from both ends of the Fed spectrum - the doves who want more easing to bring down unemployment, and the hawks who worry that more easing could overheat the economy and spark inflation.
　　"Everybody likes tying it to economic conditions," said Paul Ashworth, chief U.S. economist at Toronto-based Capital Economics. He estimates the program could eventually grow to between $960 billion to $1.44 trillion in size.
　　"The doves like this because they think the markets will think it's even bigger" than QE2, he said. At the same time, "it gives the hawks something, that if the economy picks up, they can get this stopped."
　　The Fed did not say how big it expects its latest program to be, but the clues are plain in its quarterly economic forecasts, also published Thursday.