By John S. McClenahen There's still another reason to believe that the economic recession that began in the U.S. last March will prove to be shallower and even shorter than most post-World War II downturns. Although sales of existing single-family homes dipped 0.8% in December, sales of 5.25 million homes for full-year 2001 was a new annual record, says the National Association of Realtors, Washington. The median existing-home price in the U.S. was $151,400 in December 2001, up 8.4% from December 2000. "The housing sector continued to defy the recession," observes Karen Dexter, an economist at Merrill Lynch & Co., New York. "This is an unprecedented performance, given that the housing sector is usually the first to collapse during a recession."