Standard & Poors released the August 2010 S&P/Case-Shiller Home Price Index, the most widely used US home price metric, this morning. Both the 10- and 20-city indices show a slight dip from July 2010  levels, but are stronger than the August 2009 rates– up 2.6% for the 10-city index and up 1.7% for the 20-city.

Case-Shiller computes individual metro-area indexes and calculates a weighted average to determine the national 10- and 20-city indices. Only five of the 20 index cities reported month-over-month gains from July 2010-August 2010: Chicago, Detroit, Las Vegas, New York, and Washington DC, however all of those gains were under 1%. Eight of the 20 index cities reported gains from August 2009, but those were big enough cities and big enough gains to generate growth in the national indices.

Zillow’s Home Value Index (ZHVI) is a comparable index of metro-area and national home prices and reports slightly different findings for August 2010.  The ZHVI shows a fall of 3.8% in prices nationwide from August 2009-August 2010 and a 0.3% drop month-over-month.

The Case-Shiller indices are calculated using sale price data, but control for the quality of the homes in the market so as not to fluctuate too much if there’s a change in the mix of what’s being sold in a given month. By contrast, the ZHVI takes the median Zestimate in the US or for a given metro area. The differences in methodologies likely drive the differences in results.