A new report from real estate data analysis firm CoreLogic says 1.6 million units remained in the nation’s "shadow inventory," as of January, a six-month supply.

The shadow inventory is approximately half the size of all visible inventory listings, the company said. In other words, for every two homes available for sale, there is one home in the "shadows."

CoreLogic defines shadow inventory as homes headed to foreclosure or otherwise held off the market, but which will need to be sold in coming months.

The number was about the same as it was in October 2011, the last time the firm reported a figure. On a year-over-year basis, shadow inventory was down from January 2011, when it stood at 1.8 million units, about eight-months’ supply. The flat level of the inventory is due to the flow of new seriously delinquent loans being offset by an equal amount of sales, the firm said.

"Almost half of the shadow inventory is not yet in the foreclosure process," said Mark Fleming, chief economist for CoreLogic. "Shadow inventory also remains concentrated in states impacted by sharp price declines and states with long foreclosure timelines."

The 1.6 million properties the firm counts as shadow represents about half of the total pool of 3 million properties currently considered seriously delinquent, in foreclosure or classified as REO, according to CoreLogic’s calculations. Half of the shadow inventory is comprised of seriously delinquent loans, 410,000 are in some stage of foreclosure, and 400,000 are already in REO.

"The shadow inventory remains persistent even though many other metrics of the housing market show signs of improvements," said CoreLogic President and CEO Anand Nallathambi said in a statement. "In some hard-hit markets the demand for REO and distressed property is now outstripping supply. As we move into what is traditionally the peak selling season for real estate, servicers will certainly be watching closely to see if now is the time to move more inventory out of the shadows."