That means half were within 14 percent of final sale prices for homes, while those for the other
half of homes were off by more than 14 percent — a big discrepancy for the real estate business.
The best-known of these tools — the Zestimate, from the online real estate website Zillow — began on the internet 11 years ago
and has since amassed a huge audience of homeowners, shoppers and nosy neighbors.
Unlike Zillow, Redfin is a real estate brokerage, employing agents to buy and sell homes for clients, which means
that its chief executive, Glenn Kelman, regularly hears from Redfin agents who either love its estimating tool or despise it.
That means that they shouldn’t be used as more than a conversation point between homeowners, shoppers
and their real estate agents when they’re coming up with listing prices and offers for homes.
Through refinement of its algorithms and better data about homes, Zillow, now with 110 million
homes accessible on the site, has a median error rate of around 5 percent, Mr. Humphries said.
Redfin, an internet real estate firm in Seattle, introduced its own home value estimator less than two years ago, commissioning an independent study
that showed its figures were more accurate than Zillow’s (Zillow disputes its conclusions).