Profile picture for Cahillr2

Why did the data change in Zillow

I purchased my home in June 2009. At the time of the purchase the Zestimate was $150,000. This information was used b both the seller and myself to agree upon the final purchase price which ended up being the Zestimate price. This section of the graph is now altered. It appears that data is now missing and an interpolation has been added which simply connects a previous data point and a point after with a straight line. I would like to have the original line reinstated so it coincides with the my purchase.
  • October 19 2011 - Center Square
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Answers (6)

Profile picture for blue screen exile
The chart format just changed this week (apparently last night).
(It loads much faster now, and the mouse over for the data is much easier to read and much easier to position).


And with the change, the data reference page also changed.  You now need an additional parameter:
&format=json

Without it, the page is completely blank.  I don't know if there are other options for that parameter or not.
  • October 21 2011
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Profile picture for blue screen exile
By the way, out of 636 Zestimate trends that I have checked recently, 21 of them were missing data points. (3.3%).  Thus it is not at all unusual for the charts to be missing data points.  It is "normal".  It is not a "bug".
  • October 19 2011
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Profile picture for blue screen exile
Also, if there were insufficient recent sales in an area, there was no way to calculate an estimate, thus the estimates are simply skipped when there were no sales.
  • October 19 2011
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Profile picture for blue screen exile
The data is "gone".... there is no way to get it back.

First, as already mentioned, the calculations were completely redone from January 2006 forward, so any curves viewed prior to June 2011 were from a different method.  As the calculations require renting cloud space on other machines, there is no way to get those numbers back.

SECOND, a number of people noticed spikes in the data with the new methods, sometimes due to including sales transactions that were bulk sales, or developers, or similar.  As the modeling is a learning model that takes up substantial space and resources, when false/misleading data points are noticed due to faulty input data from the county, there is no way to go back and recalculate those data points, thus for the corrections, the data points are simply omitted.

The terms of use and disclaimers specifically state the Zestimates are NOT to be used for purchase prices nor offers, but the numbers are just a starting point for further evaluation.

With the large number of spikes, Zillow has written additional software that checks the data trends and specifically removes data-points that were sharp spikes.  That data is "gone".  There is no way to get it back.  If you didn't save an image, there is no recalling that data.

You would hope that Zillow ran some archival backups of their site prior to replacing so many data points.  It is not clear if they did this or not.  Many have requested a link to archival data.  So far, Zillow has not offered to do that, and it is questionable whether they have the staffing resources to be able to do that even if they did archive it.

Just for your future reference, it is possible to get the data underlying the charts, for further evaluation and your own reference data saves (since it is much harder to use the data in image format for additional calculations).

The web address for the Zestimate data is:
http://www.zillow.com/ajax/homedetail/HomeValueChartData.htm?mt=1&zpid=#

Where the # is replaced with the Zillow property Identification number, as found on the home details page as well as in the URL address for the home details page.

For the other chart data options, the mt parameter may be changed as follows:
mt=2  % change
mt=3  listing price
mt=4  tax assessed value
mt=5 property tax
mt=6 page views (per day, one month)
mt=9 rent-zestimate
  • October 19 2011
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Profile picture for Cahillr2
I appreciate the response but it still doesn't explain why the data is missing. There are no values for this flat piece of the curve. If your scroll over their anr typically number for the city, zipcode, and your home's zestimate However, for this section of the line the zestimate is totally missing.
  • October 19 2011
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Profile picture for wetdawgs
In the middle of June 2011, Zillow rolled out a new Zestimate algorithm and recalculated historical values back through 2006  (with plans to go back further historically).    Here is a link to their FAQ.

  • October 19 2011
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