The United States officially entered recession eight months ago, according to members of the National Bureau of Economic Research - considered the official arbiter of recessions in the United States.

The group, based in Massachusetts, met yesterday to finalise its conclusions, and a public announcement is expected next week confirming that the longest American expansion on record gave way to recession in March.

Most economists define a recession as two successive quarters of economic contraction, but the Bureau uses a more complex definition.

It defines a recession as "a significant decline in activity spread across the economy, lasting more than a few months, visible in industrial production, employment, real income and wholesale-retail trade".

The Bureau maintains that "it's more accurate to say that a recession is a period of diminishing activity rather than diminished activity".

"We identify a month when the economy reached a peak of activity and a later month when the economy reached a trough. The time in between is a recession, a period when the economy is contracting."

The determination that a recession is under way is made by the Bureau's Business Cycle Dating Committee.

The committee will not declare it over until it has accumulated data over several months.

The US economy contracted in the third quarter and is expected to register negative growth in the October-December period.

The Bureau points out, however, that GDP is continually revised, often decades later, and is therefore not the best guide.