As of the fourth quarter of 2007, the S&P/Case-Shiller national index was down 10% from its peak, and an index of ten large cities had fallen by almost 16% by February. Although the Case-Shiller figures are not perfect—they miss many rural areas—they are a better gauge of price declines in big cities.

Assessing how much further house prices are likely to fall gets even trickier. One route is to look at market expectations: investors expect a further 20% drop, judging by the prices of futures contracts linked to the Case-Shiller 10 city index. But the futures market is small and illiquid and may overstate the possible declines.
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Using a model that ties house prices to disposable incomes and long-term interest rates, analysts at Goldman Sachs reckon that the correction in national house prices is only halfway through. They expect an 18-20% correction overall, or another 11-13% decline from today’s levels. But their models suggest that six states - Arizona, Florida, Virginia, Maryland, California and New Jersey, could see further price declines of 25% or more.

Optimists dispute this gloomy assessment, pointing out that some measures of housing affordability have dramatically improved. According to NAR figures, monthly payments on a typical house with a 30-year mortgage and 20% downpayment were 18.5% of the median family’s income in February, down from almost 26% at the peak - and close to the historical average. But this measure of affordability is misleading, not least because credit standards have tightened so much.
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Given the typical pace of rental growth, Mr Feroli reckons house prices (as measured by the Case-Shiller index) need to fall by 10-15% over the next year and a half for the rent/price yield to return to its historical average.