After posting several months of improvements, home prices weakened in August, according to Standard & Poor’s Case-Shiller home price indices released Tuesday.

David M. Blitzer, chairman of Standard & Poor’s index committee, called it “a disappointing report….the housing market seems to have stabilized at new lows. At this time, it does not seem that any of the markets are hanging on to the temporary momentum caused by the home buyers’ tax credits.”

The 10-city index, which includes Boston, Chicago, Denver, Las Vegas, Los Angeles, Miami, New York, San Diego, San Francisco, and Washington, posted a 0.1% monthly decline and just a 2.6% annual gain. The 20-city composite, which incorporates the 10 cities above as well as Atlanta, Charlotte, Cleveland, Dallas, Detroit, Minneapolis, Phoenix; Portland, Ore.; Seattle, and Tampa, reported a 0.2% monthly slip and a 1.7% annual gain.

The reason for the poor monthly index numbers was a broad decline in home prices across the country. Fifteen of the 20 major housing markets tracked by Case-Shiller saw values sink in August, and the five that did improve posted just “marginal improvements” of less than 1%.

Those markets with small gains were Chicago, Las Vegas, New York, and Washington, D.C.; and Detroit, which posted the top performance with an admittedly small 0.5% improvement.

Of Case-Shiller’s 20 markets, Phoenix saw the greatest monthly slide, with a 1.3% decline in home prices.

In contrast, the Federal Housing Finance Agency (FHFA) home price index posted a small monthly improvement of 0.4% for August.

This often occurs becuase the Case-Shiller and Federal Housing Finance Agency (FHFA) indices are calculated differently. Case-Shiller focuses on the 20 largest metro housing markets (which are typically more expensive markets) and includes homes purchased with jumbo loans, which can result in more volatile—or at least different--home price changes than the FHFA index.

FHFA’s figures are based on the purchase prices of homes with loans owned or guaranteed by government-sponsored enterprises (GSE) Freddie Mac and Fannie Mae in all 50 states, including home purchases in rural or less constrained markets. Unlike Case-Shiller, which concentrates on individual metropolitan markets, FHFA also looks at home prices by U.S. Census region, which can include several states, an amount of data that can mitigate the highs and lows of home price changes.

Alison Rice is senior editor, online, at BUILDER magazine.

Learn more about markets featured in this article: Atlanta, GA.