Internet Home Values: "guesstimates, not gospel"

How to Figure the Fuzzy Math of Internet Home Values


All of the competitors make it clear their numbers are guesstimates, not gospel. "A Trulia estimate is just that—an estimate," says a disclaimer on that site's new home-value tool. Zillow goes a step further, publishing precise numbers about how imprecise its estimates can be. And every major site urges home-price hunters to consult appraisers or real-estate agents to refine their results.

But despite the disclaimers, homeowners and real-estate agents say, many Web surfers put enough faith in the estimates to sway the way they shop and sell.


Determining a home's value has traditionally been the job of an appraiser, who gathers data on recently sold homes and compares them with the "subject property" to arrive at an estimate.

In the late 1980s, economists started developing automated valuation models, or AVMs, computer models that could analyze data about comparable sales, square footage, number of bedrooms and the like, in a matter of seconds. For years, these tools were mostly reserved for in-house analysts at lending banks.

It wasn't until 2006 that Zillow took them to the masses, with its Zestimates, which now offer values for more than 100 million homes based on the company's own algorithms. "Humans don't make these decisions," says Stan Humphries, chief economist at Zillow.


But appraisers and real-estate consultants say the online models can veer off target with alarming frequency. Most data for the models come from two sources: records from tax assessors and listing data for recent sales. Collection is a challenge, however, because not every county tracks properties the same way—some calculate home size by number of bedrooms, others by overall square footage. And automated models aren't designed to account for the unique construction details that often make or break a deal, or for intangible factors like a neighborhood's gentrification.


Zillow surfers who read the "About Zestimates" page find out that the site's overall error rate—the amount its estimates vary from a homes' actual value—is 8.5%, and that about one-fourth of the estimates are at least 20% off the eventual sale price. In some places, the numbers are far more dramatic: In Hamilton County, Ohio, which includes Cincinnati, it's 82%.

Comment: I'm glad I am not selling right now. And when we do I will use a realtor who is familiar with my neighborhood.

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