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Redfin has been lurking in the shadows for the past several months as online real estate companies like Trulia and Zillow grab headlines with their stock moves, lawsuits and expansion plans.

But Seattle-based Redfin today is dropping a bomb shell on the industry, commissioning a study by the WAV Group which essentially says that the big online real estate portals like Trulia and Zillow lack listings and show out-of-date information.

Some of the results aren’t too surprising. For example, the study indicated that Redfin, Windermere and Long & Foster — the three real estate brokerage firms covered in the study — show 100 percent of agent-listed homes for sale. That compares to an average of about 80 percent for Zillow and Trulia.

It’s widely known in the industry that Trulia and Zillow don’t have comprehensive databases of homes, in part because they aren’t members of multiple listing services which supply real-time data. But Redfin CEO Glenn Kelman notes that the fact is not widely known by the general public, and that’s one of the reasons why they commissioned the study.

Other results indicate that Trulia and Zillow are showing out-of-date information, including homes that are listed for sale that are no longer on the market. Thirty six percent of listings on the national portal sites appear as active, when in fact they are not, according to the study.

Redfin said that the data — which included a study of 6,000 listings in 33 zip codes — was confirmed by independent analysts.

“The findings are clear: real estate brokerage websites showed by far the most homes for sale, recognized which homes were no longer for sale, and displayed new listings much earlier,” said WAV Group CEO Victor Lund.

The study comes at a tough time for Zillow, which has seen its stock fall in recent weeks after a stock analyst questioned the viability of its business model and the SEC inquired about its revenue calculations. Shares of Zillow are still trading at about $40 per share, about double its initial public offering price from July 2011.

UPDATE: We asked Zillow for a comment on this study and here’s their statement:

“There is no gold standard for listings data, so comparing Zillow’s MLS-only listings to an MLS isn’t going to give you the whole picture. For example, Zillow has hundreds of thousands of for-sale-by-owner, new construction and foreclosure listings, which often aren’t listed on an MLS.

In addition to these listings, home shoppers visit Zillow for deep info on all homes, Zestimates, price cuts, community, rental listings and historical home information, all of which typically can’t be found on a brokerage site.

More than 36 million users came to Zillow’s site and mobile apps to research homes in August.”

RelatedZillow CEO Spencer Rascoff: Startups that try to disrupt real estate commissions are doomed to fail

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