The Federal Reserve says it will keep buying bonds until the labor market has "improved substantially," without defining the phrase. Officials may have adopted a threshold nevertheless, say two former Fed economists.

Chairman Ben S. Bernanke needs to see four months of job growth averaging at least 200,000 to justify reducing the pace of asset purchases, according to Vincent Reinhart, a former director of the Fed's Division of Monetary Affairs. Roberto Perli, a former researcher in the division, said the central bank would need to see that pace "through the summer."

"They would see that as confirmation that the economy is on a self-sustaining trajectory and they would thus be confident that they could reduce the pace" of quantitative easing, said Perli, a partner at Cornerstone Macro LP in Washington.