A Wall Street Journal analysis of 1,000 recent home sales shows that Zillow’s “Zestimates” often are very good, frequently within a few percentage points of the actual price paid. But when Zillow is bad, it can be terrible — off the mark by more than 25% on one in 10 homes. In one case it was off by $2 million. Zillow, based in Seattle, operates a Web site that offers free estimates and other online tools for real-estate buyers and sellers. It draws revenue from online advertising.

Zillow’s estimates come from a proprietary computer program that takes into account sale prices for nearby homes that appear comparable, the size and other physical attributes of the home, its past sales history and tax-assessment data, says Stan Humphries, vice president of data and analytics. Zillow tends to work best for midrange homes in areas where there are a lot of comparable houses, he says. It is less accurate for low- and high-end homes because there are fewer of those and thus less data available from comparable sales, known as “comps.” Values of rural homes are hard to gauge for the same reason. Partly for that reason, none of the Web sites can offer 100% coverage of U.S. homes; Zillow says it has estimates on about 57% of all homes.

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