Profile picture for MB01

How can you trust anything Zillow has on the site, when it changes its own history?

Zillow changed values of houses it had from years ago, to make it look to coincide with the economy as a whole.

This is a total loss for credibility and trust for Zillow.

Stick to your guns Zillow, or dont make up zestimates you have no trust in.
  • November 16 2010 - US
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Answers (5)

Profile picture for MB01

What does any improvement in their current model they have for their zestimates have to do with the fact that they changed values they had in the past.  This was done retroactively, like this is what they had stated all along.  Forget that it is in the realm of lying, if that is not that big of a deal to you, but what if you are looking at trends?  Previously with what they had, the trend of a property was that it was coming down in value by 25% in the last 6 months.  Now, even though the recent values are not changed, and because past values are changed, it is as if values went up 10% in the past 6 months.

So again, my question:  How can you have any trust in Zillow?

  • November 16 2010
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Profile picture for blue screen exile
I was not aware that they did a retroactive recalculation again recently.   With the changes they made in late 2007, they did do a retroactive change in the calculations so that there was trend consistency and not discontinuity.  This was a major change in calculation approach (taking into account local property tax assessment rules and other local factors that affect value calculations).  They debated long and hard over whether to recalculate the history or not, and ultimately decided that people looking for history trends needed the revised numbers, not the prior calculations, thus only those that saved trend images keep a record of the prior calculation results.

Zillow clearly explained their reasoning on the decision at that time.  You could probably find it with a search.

I'm not noticing any specific issues in their calculations now that indicate they have done a retroactive recalculation again.  But you do need to know that they calculate estimates 3 times a week automatically, but that much of the sale data takes a while to come in, and they now only show once a month numbers on the trend charts, so a lot of the false fluctuations now disappear, and I'm sure they correct the numbers for recent sales if new data from the county records office show up, but that would be at most 3 months back calculations.

Everyone has always known that the wild fluctuations show was only random noise due to small sample size of what sells at any given point in time, so showing fewer points to smooth out those meaningless fluctuations only seems prudent.

As for trust; I know the methodology and I know how to do the math, and I can use their access to the county records to check their work.  And they provide the tolerance range on their calculations, and the reasons for the given results is easy to verify.  Any improvements to the tolerance range and trend data is appreciated.
  • November 16 2010
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Profile picture for blue screen exile
And yes, I do have evidence that they used to calculate my home at about $1 million, due to idiosyncrasies of proposition-13 that were not properly taken into consideration at the time.  Anyone could have told you the machine method for that case was not very useful, so of course they fixed it.
  • November 16 2010
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As usual, Pasadenan knows the full, rich history.  Yes, we have only done one wholesale re-computation of historical Zestimates across our entire data footprint and that was in early 2008, associated with a major change in the valuation algorithms.  Since 2008, we have done smaller historical revisions in only three specific regions in order to address issues that arose there (almost always because of changes to underlying data, not to our algorithms) and these have been in the Detroit metro, Clark County NV, and Merced County CA.

If you are located in one of these three regions and have noticed a historical revision, apologies if this created concerns for you.  If you are not in one of these regions, we have not re-computed historical Zestimates so please detail the issues that you are seeing so we can investigate. 

As is implied in Pasadenan's post, we will do future re-computations around major algorithmic changes, but we will notify our users when we do so.  We do not ever change individual Zestimate time series because of individual property-level inaccuracy. 

Hope this helps.
  • November 19 2010
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Profile picture for blue screen exile
Want to see how the 2007-08 recalculation affected long held properties that were under prop-13 rules?

For 2007, the Zillow trend:


Now (5 yr trend)- Same property:


(Obviously, the historic recalculation was "helpful" and worthwhile).

(Notice, that the 2007 saved image was just as the bubble decline was starting to affect the area).
  • November 22 2010
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