Economists say Fed’s stimulus plan has had mixed results

But most Americans are not feeling the difference, in part because those benefits have been surprisingly small. The latest estimates from economists, in fact, suggest that the pace of recovery from the global financial crisis has flagged since November, when the Fed started buying $600 billion in Treasury securities to push private dollars into investments that create jobs.

The Fed’s policy-making board will meet Tuesday and Wednesday, after which the Fed chairman, Ben S. Bernanke, will hold a news conference for the first time to explain its decisions to the public. But a broad range of economists say the disappointing economic results show the limits of the central bank’s ability to lift the nation from its malaise.

“It’s good for stopping the fall, but for actually turning things around and driving the recovery, I just don’t think monetary policy has that power,’’ said Mark Thoma, a professor of economics at the University of Oregon, referring specifically to the bond-buying program.

Bernanke and his supporters say the purchases have improved economic conditions, all but erasing fears of deflation, a pattern of falling prices that can delay purchases and stall growth. Inflation, which is beneficial in moderation, has returned to healthy levels since the Fed started buying bonds.

“These actions had the expected effects on markets and are thereby providing significant support to job creation and the economy,’’ Bernanke said in a February speech, an argument he has repeated frequently.

But growth remains slow, jobs remain scarce, and, with the debt purchases scheduled to end in June, the Fed must now decide what comes next.

The Fed generally encourages growth by pushing down interest rates. In normal times, the central bank reduces short-term interest rates and the effects spread to other kinds of borrowing like corporate bonds and mortgage loans. But with short-term rates hovering near zero since December 2008, the Fed has tried to attack long-term rates directly by entering the market and offering to accept lower returns.

A growing body of research suggests that the Fed could have a larger impact by spending more money on a broader range of debt, like mortgage bonds. The Fed limited the program to $600 billion under considerable political pressure. While that sounds like a lot of money, the purchases have not even kept pace with the government’s issuance of new debt, so that in key respects the effort has amounted to treading water.

A vocal group of critics, meanwhile, argues that the Fed has already done far too much, amassing a portfolio of more than $2 trillion that may impede the central bank’s ability to curb inflation. Some of these critics view the rising price of oil and other commodities as harbingers of broader price increases.

“I wasn’t a big fan of it in the first place,’’ said Charles I. Plosser, president of the Federal Reserve Bank of Philadelphia and one of the 10 members of the Fed’s policy-making board. “I didn’t think it was going to have much of an impact and it complicated the exit strategy. And what we’ve seen has not changed my mind.’’

The Fed’s decision to buy bonds, known as quantitative easing, emulated Japan’s central bank, which started buying bonds in 2001 to break a deflationary cycle.

The American version worked well at first. Between November 2008 and March 2010, the Fed bought more than $1.7 trillion in mortgage and Treasury bonds, holding down mortgage rates and reducing borrowing costs for well-regarded companies by about half a percentage point, according to several studies. That is an annual savings of $5 million on every $1 billion borrowed.

As the economy sputtered last summer, Bernanke indicated in an August speech at a Wyoming resort that the Fed would start a second round of quantitative easing, soon nicknamed QE 2. The initial response was the same: Asset prices rose, interest rates fell, and the dollar declined in value.

But in addition to being smaller, and solely focused on Treasury bonds, there also was a problem of diminishing returns. The first round of purchases reduced the cost of borrowing by persuading skittish investors to accept lower risk premiums.

With markets closer to normalcy, Bernanke warned in his August speech that it was not clear that the Fed would have comparable success in persuading investors to accept even lower rates of return.

“Such purchases seem likely to have their largest effects during periods of economic and financial stress,’’ he said.