SOUNDING more like a cartographer than a central banker, Ben Bernanke this week showed off the Federal Reserve's latest gizmo for tracking America's property bust: a series of maps that colour-code price declines, foreclosures and other gauges of housing distress for every county in the country. The Fed chairman's goal was to show graphically that falling prices meant more foreclosures, and he went on to urge lenders to write down the principal on troubled loans where the house is worth less than the value of the mortgage. But the jazzy design of his maps—where hotter colours imply more trouble—also makes a starker point. The pain of America's housing bust varies enormously by region. Hardest hit have been the “bubble states”—California, Nevada and Florida, as well as parts of the industrial Midwest. The biggest uncertainty hanging over the economy is how red will things get.

The answer is not simple. For a start, it is hard to be sure just how much house prices have fallen. America has several house-price indices and they tell different stories. Widely cited, but least useful, are monthly figures showing median home prices produced by the National Association of Realtors (NAR). These indicate that median prices are down some 13% from their peak, but since these averages do not adjust for the mix of homes changing hands, which fluctuates from month to month, they are inevitably distorted.

Mr Bernanke's maps use figures from the Office of Federal Housing Enterprise Oversight (OFHEO). Its statistics have broad geographic reach and track repeat sales of the same house. The monthly national index suggests average prices have fallen only 3% from a peak in April 2007, and the quarterly figures are still positive (see left-hand chart). But OFHEO's figures include only houses financed by mortgages backed by the government-sponsored giants, Fannie Mae and Freddie Mac. By excluding subprime and jumbo loans, they leave out the top and bottom of the market—where prices rose fastest during the bubble and where the mortgage mess was most severe. Thus OFHEO's figures probably understate the scale of the housing mess, particularly in states such as California and Florida. Another set of indices, developed by Robert Shiller and Karl Case and produced by Standard & Poor's (S&P), a rating agency, includes all types of houses and, not surprisingly, show house prices rising faster during the boom and falling faster now. 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.

The discrepancy between supply and demand suggests that prices could fall a lot more. By historical standards there is a huge glut of unsold homes on the market. The homeowner-vacancy rate—which includes all vacant homes for sale—has soared to a record level of 2.9%, which means that there are some 1.1m “excess” houses for sale compared with the average between 1985 and 2005. Although the inventory of new homes is falling as builders have slashed their production, the supply of homes for sale is being pushed up by foreclosures even as demand from new homeowners remains weak.

By most measures, prices are still above the levels implied by the fundamentals. 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. The latest survey of loan officers conducted by the Fed suggested on May 5th that 60% of banks tightened their lending standards for prime mortgages in the first three months of 2007. And, as Michael Feroli of JPMorgan points out, the affordability gauge depends on what measure of home prices you look at. Use the Case-Shiller index, where the affordability of housing worsened sharply during the boom, and mortgage payments are still high in relation to incomes.

The right-hand chart shows a better measure of housing fundamentals—the relationship between house prices and rents. This is a sort of price/earnings ratio for the housing market: the price of a house reflects the discounted value of future ownership, either as rental income or as rent saved by an owner who lives in the house.

A recent analysis by Morris Davis of the University of Wisconsin-Madison, and Andreas Lehnert and Robert Martin of the Fed, shows that the rent/price yield in America ranged between 5% and 5.5% from 1960 to 1995, but fell rapidly thereafter to reach a historic low of 3.5% at the height of the boom. 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. Again, that suggests the national housing bust is only halfway through. And, given the scale of excess supply, house prices—particularly in hard hit areas—are likely to overshoot. All told, Mr Bernanke's maps are going to get a lot redder—and the pressure on policymakers to help struggling homeowners is bound to increase.