Amid an unusual level of uncertainty about its intentions, the Federal Reserve's monetary policy-making Open Market Committee meets today to decide whether to raise interest rates for the third time in five months.

Futures markets in the last week have indicated a 50 per cent chance that the Fed will indeed raise its key market instrument, the federal funds rate, by a quarter of a percentage point to 5.5 per cent. And, fittingly, the gallery of Fed watchers on Wall Street is split down the middle on whether Mr Alan Greenspan and his colleagues will pull the trigger.

A survey of economists at 30 leading bond market participants late last week showed just how uncertain economists are, with 15 expecting an increase and 15 saying there would be no change.

Uncertainty before an FOMC meeting is quite rare. Indeed, it is one of the ironies of modern central banking that the Fed, which does not enjoy much of an international reputation for the openness of its internal procedures, is probably the most forthright in informally disclosing its intentions to financial markets.

A decade ago, a senior Fed official coined the phrase "constructive ambiguity" to describe the appropriate central bank approach to policy-setting. Every time Mr Greenspan speaks, the broad thrust of his remarks conforms almost perfectly to that philosophy. Last autumn's rate cuts was widely flagged - in fact, it turned out to be a little disappointing since the Fed's advance signalling had been so strong that some investors were expecting a half-point cut rather than the quarter-point move that came. That was followed, however, by a powerful example of the effectiveness of the surprise move - when the Fed cut rates between regular meetings of the FOMC. The third move in November, was back to the more familiar pattern of being pre-signalled.

But in the run-up to the current meeting, Mr Greenspan has given nothing but ambiguous signals - a paean of praise for leaps in productivity here, a warning about overly rapid growth there, but just a suggestion over yonder that growth might already be slowing.

And the economic data have continued to provide their own ambiguity - depicting an economy growing well in excess of traditional "speed limits" but still not exhibiting any clear sign of inflationary stress. What explains the Fed's reticence this time?

Some economists believe the committee is genuinely split. But it seems more likely that the committee members, and perhaps even the chairman himself, have not made up their own minds yet. The arguments for and against a rate increase seem, even to established hawks and doves in the monetary policy establishment, especially well balanced this time. In the last week alone, the data have shown that productivity is up, but demand growth remains strong - unit labour costs are down, but core wholesale prices are up.

The uncertainty, of course, carries with it a danger - whichever way the Fed moves today, markets could react aggressively, immediately jeopardising the central bank's prospects of achieving its objectives.

If there is a rate rise, markets could swoon, raising the risk of a serious correction and forcing the Fed to backtrack. Perhaps more likely, if there is no change, some analysts expect bond and equity prices to take flight, adding yet more fuel to the flames of rapid growth.

But, perhaps tellingly, Wall Street economists do not seem to be expecting much reaction either way. Though there is a risk that the official statement accompanying the rate move, and the announcement of the "bias" that says which way the Fed is leaning, could send some unsettling signals about its long-term thinking, no-one really believes the central bank is in aggressively pre-emptive mood.