CoreLogic, the Santa Ana-based analytics firm, believes existing home sales for all of 2010 may be 15% to 20% lower than reported by National Association of Realtors (NAR).
But the trade group claims any rebenching of its figures will turn up only minor changes in its sales numbers.
CoreLogic analysts estimate that repeat home sales totaled 3.6 million in 2010, down 12% from 2009. NAR reported in January that existing home sales fell only 5% in 2010 to 4.9 million.
"NAR's existing sales data are overstated by about 15% to 20%," CoreLogic said in a February report on U.S. Housing and Mortgage Trends.
"There are several reasons for the divergence, including benchmarking drift," the Santa Ana, Calif., vendor said.
NAR spokesman Walter Molony said the trade group has had discussions with CoreLogic. "We are working with outside housing economists on the methodology to determine if there is any drift in the data and by how much," he said. "At this point we believe any data drift will be relatively minor," he said.