NEW YORK (CNNMoney) -- If you thought the U.S. housing market couldn't get much worse, think again.
Far fewer homes have been sold over the past five years than previously estimated, the National Association of Realtors said Tuesday.
NAR said it plans to downwardly revise sales of previously-owned homes going back to 2007 during the release of its next existing home sales report on Dec. 21.
NAR's existing home sales numbers, released monthly, are a closely followed gauge of the health of the housing market.
While NAR hasn't revealed exactly how big the revision to home sales will be, the agency's chief economist Lawrence Yun said the decrease will be "meaningful."
"For the real estate business, this means the housing market's downturn was deeper than what was initially thought," Yun said.
Yun said the database NAR uses to track existing home sales, the Multiple Listing Service (MLS), has led the real estate agency to over-count existing home sales for several reasons.
The MLS database only includes home sales listed by realtors, and excludes homes listed by owners, providing a very narrow view of the market. And because more people are using realtors to list their homes instead of selling them independently, realtor-listed sales numbers have become artificially inflated, said Yun.
In addition, some of the assumptions NAR used in calculating its data have become outdated, since they were based on 2000 Census data.
The MLS has also been expanding its geographic coverage, so it may have appeared that there were more home sales simply because data from new areas were starting to show up. Also because of this geographic expansion, the system has been double-counting sales of some homes that can be considered part of multiple regions.
"Colorado Springs has their own database, but because the Denver market is nearby they may also list that home in the Denver database, so when the home gets sold, both Denver and Colorado Springs will say sales rose -- so that's genuine double-counting," said Yun.
Yun said NAR realized this upward "shift" in data during its most recent re-benchmarking process this year. With the help of the government, economists and other real estate groups, NAR has now taken these factors into account and will issue revised numbers on Dec. 21 at 10 a.m.
"There are multifaceted reasons why things were drifting upward in our database," said Yun. "We have tried to adjust for all these factors so that we have a better understanding of total home sales in America."
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