WASHINGTON - It was big news last week when the Federal Reserve announced that it wants to maintain its current low-interest rate policy until unemployment, now 7.7 percent, drops to at least 6.5 percent.
The Fed was correctly portrayed as favoring job creation over fighting inflation, though it also set an inflation target of 2.5 percent
What was missing from commentary was caution based on history: The Fed has tried this before and failed - with disastrous consequences.
By "this," I mean a twin targeting of unemployment and inflation.
In the 1970s, that's what the Fed did. Targets weren't announced but were implicit.
The Fed pursed the then-popular goal of "full employment," defined as a 4 percent unemployment rate. Annual inflation of 3 percent to 4 percent was deemed acceptable.
The result was economic schizophrenia. Episodes of easy credit to cut unemployment spurred inflation, which inspired tighter credit that boosted joblessness.
By 1980, inflation was 13 percent and unemployment, 7 percent.
The Fed was in over its head. It didn't know enough to do what it (and many others) thought it could do.
Today's problem is similar. Although the Fed has learned much since the 1970s - including the importance of low inflation - its economic understanding and powers are still limited. It can't predictably hit a given mix of unemployment and inflation.
Striving to do so risks dangerous side effects, including a future financial crisis.
For proof of the Fed's limits, look to the Fed itself. Since the 2008-09 financial crisis, which the Fed didn't anticipate or prevent, it has repeatedly miscalculated.
It's made heroic efforts to revive the economy, including keeping short-term interest rates near zero since late 2008 and pumping out more than $2 trillion by buying mortgage bonds and U.S. Treasury securities.
But as Chairman Ben Bernanke conceded last week, the Fed has consistently overestimated the recovery's strength. Even if the Fed's policies were right, their impact has been exaggerated.
Throwing money at the economy has produced only modest gains. The money paid out to buy bonds has aimed, through reinvestment in the stock and bond markets, to boost stock prices and lower interest rates on other bonds.
These changes are intended to stimulate spending. Many economists agree that more can be done.
"Is the Fed running out of steam? To some extent," says Mark Zandi of Moody's Analytics. "But interest rates on 30-year fixed mortgages are 3.35 percent. They could be lower."
What might doom the Fed's ambitions?
One threat is irrelevance. Credit is arguably so easy that the Fed can't do much more.