The National Association of Realtors said a benchmarking exercise had revealed that some properties were listed more than once, and in some instances, new home sales were also captured.

"All the sales and inventory data that have been reported since January 2007 are being downwardly revised. Sales were weaker than people thought," NAR spokesman Walter Malony told Reuters.

"We're capturing some new home data that should have been filtered out and we also discovered that some properties were being listed in more than one list."