Editor’s note: This story has been updated with comments from Zillow and Trulia, and additional details on ZipRealty’s study of listings accuracy.

Following in the footsteps of Redfin, ZipRealty Inc. is emphasizing the timeliness and accuracy of the Internet Data Exchange (IDX) listing data it serves up on its website in comparison to the sometimes dated and incomplete information on third-party listing portals like Zillow and Trulia.

ZipRealty is offering a "Listing Check" tool that it says will allow consumers to use its website to double-check whether a home they’ve seen listed for sale on another site is actually for sale.

The Listing Check tool is available in 34 markets where ZipRealty has access to IDX listings. The company operates as a brokerage in 19 markets, and is also able to display IDX listings in 15 other "Powered by Zip" markets where it provides leads and customer relationship management tools to other brokerages.

Websites operated by real estate brokerages and agents receive listings directly from multiple listing services, which provide IDX listing feeds to members. The feeds include all listings represented by participating brokerages in a given market.

Thanks to its ties to the National Association of Realtors, Realtor.com gets listings directly from most of the nation’s more than 900 MLSs.

Other third-party listing portals, like Zillow and Trulia, receive some of their listings directly from MLSs. But brokers have the right to withhold listings they represent from the portals, and in many markets, portals must rely on individual brokerages, agents and third-party syndicators for listing data. Difficulties in managing feeds from multiple sources means that portals sometimes have incomplete or inaccurate listing data.

Brokerages in "Powered by Zip" markets can withhold listings from third-party websites like Zillow and Trulia, but cannot prevent their listings from appearing on ZipRealty.com without withdrawing entirely from IDX agreements.

To emphasize the accuracy of the IDX listing data displayed on ZipRealty.com, the company says it analyzed listings published by Zillow and Trulia in 50 ZIP codes across the following 13 major metropolitan areas: Boston, Chicago, Dallas, Denver, Houston, Las Vegas, Los Angeles, Philadelphia, Portland, the San Francisco Bay Area, San Diego, Seattle and Washington, D.C.

In the areas studied, ZipRealty claims that more than 15 percent of homes shown for sale on Zillow and Trulia weren’t actually on the market, and up to 30 percent of homes that were listed for sale in an MLS were not identified by the portals as being on the market.

"Finding that perfect home online and then discovering that it’s already been sold presents an extremely frustrating scenario to homebuyers," said ZipRealty CEO Lanny Baker in a statement. "Other major online real estate websites continue to show homes listed as ‘for sale’ for several days — even weeks — after they have sold, and no homebuyer wants to waste time chasing after properties that are already off the market."

Summary of ZipRealty study findings

Site % of listings on the site versus the MLS % of listings on the site listed as for sale while not listed for sale in the MLS
Zillow 73% 16%
Trulia 78% 17%
ZipRealty 99% 1%

Source: ZipRealty

ZipRealty looked at a control set of 2,981 homes actively listed for sale in the 16 MLSs in the markets above on Oct. 28, 2012. That set of homes was then cross-referenced, using both the properties’ addresses and MLS listing identification numbers, with the homes listed for sale on Zillow, Trulia and ZipRealty in those markets.

If an MLS-listed home was not found on one of the three sites, ZipRealty manually double-checked for incorrect information and updated its results as needed.

For homes listed for sale on the three sites that were not listed for sale in the MLS, the properties’ historical information in the MLS was looked at.

"In many cases," the report noted, "these ‘extra’ homes shown within the portals’ home listings results reflected listings that had previously expired, sold or otherwise been deactivated within the MLS database while still shown as ‘for sale.’"

Last fall, technology-based brokerage Redfin hired a consulting firm, WAV Group, to compare the accuracy of listing data published by Zillow and Trulia in 11 major markets with IDX listing data displayed by Redfin and two other brokerages.

The WAV Group study found that about 36 percent of the listings shown as active on Zillow and Trulia were no longer for sale in the local MLS, while brokerage websites had few or no such errors.

Trulia and Zillow, which have both instituted programs to improve listings accuracy, said the WAV Group study did not tell the whole story.

Both companies said they offer tools that are often now found on broker and MLS sites that allow consumers to research market conditions. Some homes — such as those being sold by owners, and newly constructed homes — are not listed in the MLS because they are not represented by a real estate broker.

Trulia spokesman Ken Shuman said ZipRealty didn’t detail which ZIP codes were looked at in its report, or why the markets that were studied were chosen.

"I raise an eyebrow at this," Shuman said. "Did they look at all 15 ZIP codes in San Francisco. or just the four where Trulia had the most discordant match with the MLS?"

Shuman said Trulia has "invested heavily in delivering quality data to consumers," pointing out the firm’s Trulia Broker Program, which gives brokers incentives for providing a direct listing feed to the portal, and new continuously updated protocol for data it gets directly from MLSs.

Zillow has also been focused on improving listing coverage and accuracy. It announced its 49th broker in the Zillow Pro for Brokers program last month and in July it started ramping up its efforts at getting listing feeds directly from MLSs.

"What’s more important to understand about a study like this is that there is no gold standard for real estate listings," said Zillow spokeswoman Cynthia Nowak.

Nowak said Zillow has information on many listings — like for-sale-by-owner, pre-market and new construction homes — that aren’t in MLSs because they aren’t represented by real estate brokers.

Nowak also pointed out that home shoppers use Zillow for reasons beyond looking for for-sale homes, but also for "deep information on all homes, the Zestimate, price cuts, communities, rental listings and historical home information."

"If (Zillow and Trulia) are saying this isn’t right, God bless them. Let’s see the data," ZipRealty CEO Lanny Baker told Inman News. The study looked at the prime ZIP codes in the markets covered, he said.

Baker says he’s aware that consumers use real estate sites for different reason. The new ZipRealty tool and study, he said, is an effort to point out where he thinks ZipRealty excels — providing search for MLS-listed homes.

Baker said many consumers aren’t aware that popular portals like Zillow and Trulia don’t have all the for-sale listings in some markets. Studies like ZipRealty’s are meant to bring the issue to light, he said.

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