Zillow off a median of 8% on home prices; is this a big problem?

Zillow’s CEO recently discussed the error rate of his company’s estimates for home values:

Back to the question posed by O’Donnell: Are Zestimates accurate? And if they’re off the mark, how far off? Zillow CEO Spencer Rascoff answered that they’re “a good starting point” but that nationwide Zestimates have a “median error rate” of about 8%.

Whoa. That sounds high. On a $500,000 house, that would be a $40,000 disparity — a lot of money on the table — and could create problems. But here’s something Rascoff was not asked about: Localized median error rates on Zestimates sometimes far exceed the national median, which raises the odds that sellers and buyers will have conflicts over pricing. Though it’s not prominently featured on the website, at the bottom of Zillow’s home page in small type is the word “Zestimates.” This section provides helpful background information along with valuation error rates by state and county — some of which are stunners.

For example, in New York County — Manhattan — the median valuation error rate is 19.9%. In Brooklyn, it’s 12.9%. In Somerset County, Md., the rate is an astounding 42%. In some rural counties in California, error rates range as high as 26%. In San Francisco it’s 11.6%. With a median home value of $1,000,800 in San Francisco, according to Zillow estimates as of December, a median error rate at this level translates into a price disparity of $116,093.

Thinking from a probabilistic perspective, 8% does not sound bad at all. Consider that the typical scientific study works with a 5% error rate. An eight percent error rate suggests the estimate is right 92% of the time. As the article notes, this error rates differs across regions but each of those have different conditions including more or less sales and different kinds of housing. Thus, in dynamic real estate markets with lots of moving parts including comparables as well as the actions of homeowners and homebuyers, 8% sounds good.

Perhaps the bigger issue is what people do with estimates; they are not 100% guarantees:

So what do you do now that you’ve got the scoop on Zestimate accuracy? Most important, take Rascoff’s advice: Look at them as no more than starting points in pricing discussions with the real authorities on local real estate values — experienced agents and appraisers. Zestimates are hardly gospel — often far from it.

Zillow can be a useful tool but it is based on algorithms using available data.

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