Comparing Zillow and Redfin Price Estimates in Time
I wrote about the pre- and post-sale estimates of a house’s price from four online brokers a little while ago. One limitation of that experiment was that I only collected the data at a few points in time for the house: one value before the listing, one value after the listing, and a final value after the sale.
But what if the estimates change day-to-day? Christopher Moody posted this on Twitter:
I suspect that their algos factor in impressions. I bet there’s a fascinating time series in between pre- and post-listing price estimates
I decided to look.
You can find the Jupyter notebook used to perform this analysis here (rendered on Github). The data can be found here.
Data Collection
I recorded the estimated sales price everyday from Zillow and Redfin. I excluded Realtor.com and Xome because they do not update their estimate frequently (they seem to only update when the status of the house changes from pre-listing to listing to pending to sold). I occasionally missed a day of data because I was collecting it manually.
Unfortunately the idea to collect the time series came after the house was already pending, so I missed some of the most interesting changes. I have set up a Python script to scrape listings for a few other houses and should have a better time series for another article.
Plot
Here a plot comparing the estimates for the sales price from Zillow and Redfin in time:
The daily price estimates from Redfin are shown using red circles, the estimates for Zillow are shown using blue triangles.
Comments
There is a lot of movement in the estimates. Redfin and Zillow start off far apart. Redfin reverts strongly to the list price once the house if official on the market (I missed Zillow’s price on the day of listing). As the house goes pending, both estimates increase drastically.
This suggests that some impression metric is used in both models, because there is not much other information available that could cause such a large increase. It will be interesting to see what a better time series will reveal.
Both estimates vary day-to-day even after the house is pending. You would think all the information you need to determine the sale price would be present once the offer is made, but there are facts you can observe after the listing is pending—like how long the closing period is—that must have predictive power. Redfin’s estimate remains pretty tight while Zillow’s changes wildly, sometimes by 20% between days! The high variance really reduces my confidence in Zillow’s model.
Finally, both estimates completely miss the actual sales price. The estimates eventually jump to near the sales price, but it is not immediate; they take about a week to adjust. Possibly a model update is triggered immediately for newly listed properties—notice how the Redfin price adjusts the same day it was listed—but not for sales? This makes some sense. A listing is an event these business care about because they can profit off it, but a sale does not provide that opportunity.