National U.S. home sales figures will be lowered dating back to 2007 after the private trade group that collects the figures says the numbers were too high.

The National Association of Realtors says it will release the downward revisions for previously occupied homes on Dec. 21.

Among the reasons for the inflated figures: changes in the way the Census Bureau data collects data, population shifts and some sales being counted twice. Last year's total sales figure of 4.91 million was the worst in 13 years.

CoreLogic, a private real-estate data firm in Santa Ana, California, first raised doubts about the annual numbers earlier this year.

The changing numbers could impact how economists view data from the trade group. It could also affect companies who use the figures for hiring and expansion plans.