Zillow: New and improved

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News item: Zillow.com tonight announces changes it says will make its online valuation service broader and more accurate. The bigger Zillow database will now contain information on 80 million homes, up from 70 million previously. Of those 80 million homes, Zillow will publish ‘zestimates’ for 67 million.

The accuracy part: Zillow has tweaked its algorithms to incorporate more information, and believes its ‘Zestimates’ are now significantly more accurate. In Los Angeles, Zillow now measures its median error rate at 7.6% -- that is, when houses sell in L.A., the median departure from Zillow’s Zestimate is 7.6%. (There are still some pretty big mistakes: Zillow says 16% of L.A. home sales depart from the Zestimate by 20% or more.)

(Funny: people looking to buy a house generally believe Zillow’s Zestimates are too high; those looking to sell believe the estimates are too low. Zillow says its misses are evenly distributed between highballs and lowballs).

If you are a numbers geek, you might find this interesting: Zillow VP of Marketing Spencer Rascoff says greater accuracy came from a couple of sources: first, factoring in more human information, in which people ‘claim’ their home on Zillow and manually update the information. Second, instead of grouping homes by county, and assuming the same statistical trends (price per square foot, price per bedroom, etc.) hold across a county, Zillow now groups homes into much smaller neighborhood clusters of roughly 1,000 homes.

Lastly, Zillow is learning that some pieces of data have very little value, and discounting them. In California, Zillow has learned that property tax information reveals very little about the absolute, or relative, value of a home. Thanks, Prop. 13.

Your thoughts? Any other valuation tools you like? Email story tips to peter.viles@latimes.com.