"Home prices in the U.S. are likely to start to stabilize or touch bottom sometime in the first half of 2009," he said in an interview. Tracing a jagged curve with his finger on a tabletop to underscore the difficulty in pinpointing the precise trough, he cautioned that even at a bottom, "prices could continue to drift lower through 2009 and beyond."

He's relying on several data sources:

One is the supply of vacant, single-family homes for sale, both newly completed homes and existing homes owned by investors and lenders. He sees that "excess supply" -- roughly 800,000 units above normal -- diminishing soon. The other is a comparison of the current price of houses -- he prefers the quarterly S&P Case Shiller National Home Price Index because it includes both urban and rural areas -- with the government's estimate of what it costs to rent a single-family house.