Profile picture for kalvaria

Does Zillow "restate" historical Zestimates?

I've been obsessively checking the Zestimate for a house I'm i'm looking at and noticed that today, not only is the Zestimate lower, but so are the historicals.  The difference is $120k, so its not a trivial amount.  I know we shouldn't rely on a single source for these estimates, but there is no way I'm buying the Zillow home value report" they are offering if the values are going to fluctuate like this, especially historically.
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June 16 2011 - Redwood City
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Profile picture for dougtulane
Our house was zestimated at about $285k before we bought it for $265k. The day our sale posted to Zillow, the Zestimate plummeted to $255k.

Now the historical Zestimate has a much smoother curve to the sale point, suggesting to me that Zillow is retroactively fixing their zestimates to make them appear more predictive, giving prospective buyers and sellers false confidence in the accuracy of Zillow's formula. This, if true, is incredibly dishonest.

Really, whatever the reason, revising historical data points is just plain troubling.

P.S. Oh, and "it's free" doesn't cut it as an excuse. Zillow is an influence on a multi-trillion dollar industry, and they have a responsibility to their users.
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June 16 2011
Profile picture for Dunes....

"Historical Zestimate"

"With the data delivery yesterday, we have re-computed historical Zestimates back to January 2006. There are a few algorithmic developments that we are still working on and, once these are done, we intend to re-compute all Zestimates back to 1996."
http://www.zillow.com/blog/research/2011/06/14/upgrading-the-zestimate/

And....Zestimate Improvements FAQ.
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June 16 2011
ANY valuation that includes an 'algorithim', or some type of reconciliation of values and adjustments (ie an appraisal) , is inherently skewed one way or the other, period. Thats because either a computer or a person puts more or less weight on adjustments. In ZIllows case, its an algorithim, and their new algorithim is used because Zillow says it MORE accurate, hence they SHOULD be using it. Its only data, remember, not the ACTUAL value.

Dont be too flipped out on it, thats why you can have 10 appraisals on one property and have them all different.

Since Zestimates on homes are free, I wouldnt complain too much. Its free, and take it for what its worth. Your only other choice is another - equallaly falible free home value site - or a paid data service, which will cost you.

Dont 'buy' the ZIllow Home Value Report if you dont want to - get an appraisal!
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June 16 2011
The info they base the zestimate on usually take several weeks to reach their database, so sometimes it catches up in groups. On top of that, Zillow just changed their algorithm a day or two ago and it has effected historical numbers.
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June 16 2011
 
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