Housing guru Robert Shiller sees more pain ahead.
NEW YORK (CNNMoney) -- In an off-hand remark before cameras and microphones, economist and housing market guru Robert Shiller opined earlier this year that he would not be shocked if there was another 10% to 25% in the nation's home price plunge -- and he's not backing down from that statement.
At a S&P Housing Summit in New York, Shiller on Thursday reiterated his fears of falling home prices. It's not a forecast, he said, just a comment on his understanding of housing market trends.
He explained that speculative markets, like stocks or commodities, act like random walks. They go up and down all the time. Housing market direction tends to be more consistent.
"I worry that this is a real and continuing downturn, like in Japan," Shiller said. "It had a boom in the 1980s that peaked in 1991. Prices declined in the major cities for 15 straight years after that."
The U.S. housing market is hard to predict because the boom and bust it went through was unique. Shiller has studied historical price data back to the 1890s and found nothing like it.
"This is the biggest housing boom and bust in U.S. history," he said. "The bubble was unique. "That makes it impossible for statisticians to forecast because they deal with things that repeat themselves. You see a pattern and expect it to repeat."
It's even different from the Great Depression, when the home price plunge was at about the same rate. The big difference, however, was that prices of nearly everything else cratered in the 1930s as well -- which has not been true during the housing bust. 
| Overnight Avg Rate | Latest | Change | Last Week |
|---|---|---|---|
| 30 yr fixed | 3.80% | 3.88% | |
| 15 yr fixed | 3.20% | 3.23% | |
| 5/1 ARM | 3.84% | 3.88% | |
| 30 yr refi | 3.82% | 3.93% | |
| 15 yr refi | 3.20% | 3.23% |
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