A company that tracks residential “shadow” inventory reported a slight decline in January compared with the same month a year ago.

Santa Ana, California-based CoreLogic, a provider of consumer, financial and property information and business services, reported 1.8 million shadow units, representing a nine months’ supply. That number is down slightly from 2.0 million units, also a nine months’ supply, in January 2010.

“While the trend of the shadow inventory is improving somewhat, the current level and distressed months’ supply remain very high,” said Mark Fleming, chief economist for CoreLogic. “The short-term weakness in prices and longer-term weakness in the drivers that affect the housing market imply that excess supply will remain high for an extended period of time.”

CoreLogic estimates shadow inventory by calculating the number of distressed properties not currently listed on multiple listing services (MLS) that are seriously delinquent (90 days or more), in foreclosure and real estate owned (REO) by lenders.

The highest levels of distressed months’ supply, which is the ratio of the number of properties that are 90 days or more delinquent to the number of home sales, are in New Jersey, Illinois and Maryland. Washington ranked 10th. The driving force behind these states with the highest distressed supply is a combination of higher than average 90+ day delinquencies and lower sales activity.

The states with the lowest distressed months’ supply are where the boom/bust did not occur and include North Dakota, Alaska and Wyoming. The largest state with the lowest level of distressed months’ supply was Texas.