# Comparing Zillow, Redfin, and Realtor.com Price Estimates in Time

A few months ago I built a time series of a house’s price estimate from Zillow and Redfin. But there were some problems:

• I only collected dense data starting right before the property went pending.
• I collected data by hand so I often missed days.

When another nearby house put a sign out saying “coming soon”, I wrote a script to automate the scraping and collected a much denser time series from Zillow, Redfin, and Realtor.com. Let’s see what we can learn with more complete data!

You can find the Jupyter notebook used to perform this analysis here (rendered on Github). The data can be found here.

## Data Collection

I wrote a script to download the entire page for the specific house from each of the three sites. I ran it on my Raspberry Pi everyday using `cron`. I parsed the HTML using Python and a wrote the cleaned data to JSON. That parsing notebook can be found here (rendered on Github). I won’t include the raw data, you will have to collect some yourself.

## Plot

Here is 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, the estimates for Realtor.com are shown using purple diamonds.

I wrote a script to ensure that I would get data for every day, but as you can see there are still many missing points.

#### Realtor.com

Realtor.com is the most frustrating! As soon as the house was actually listed on the market they stopped providing estimates! Realtor.com starts estimating again only after the sale price is posted. This defeats the entire point! The estimate is most important when the house is actually for sale and Realtor.com just punts completely. Embarrassing.

Still we can see that, when the house is not for sale, they update their estimate roughly every two weeks. Their initially estimates are not too bad, just about 6% low from the final sale price.

#### Zillow

Zillow similarly is missing estimates for most of the time when the home is actually for sale. Their page shows “Zestimate: None” with an error explanation blaming county transactional data.1 I am sure that’s true but I am unimpressed. Dealing with missing data is a key part of building a robust machine learning model.

Zillow’s model updates more frequently than Realtor.com’s. It slowly climbs until it abruptly stops estimating a few days after the listing is posted. This suggests they make use of a different model for currently on-the-market homes and that that model requires more and different data than the off-the-market model.

The Zillow estimate does return at the end of the pending period but… I do not have anything nice to say about it. Just look at that variance!

Zillow underestimates the final price by about 10%.

#### Redfin

Redfin is the only company that keeps posting estimates once the house is actually for sale! Their pre-listing estimate is almost exactly right, but once the house is listed their on-the-market model over estimates by about 10%.

Before the listing is posted, Redfin updates its estimate roughly weekly, and like Zillow it takes 5 days to switch to the on-the-market model. This suggests that both sites get their data indicating the house is for sale from the same source. After the listing the model updates daily.

The on-the-market model trends upwards at first and then stabilizes after the house is pending, but because the time between listing and pending was so short it is impossible to tell if the stabilization was due to the listing going pending or not.

## Conclusions

With the denser data and all three sites to compare, I conclude the following:

• Zillow and Redfin both use separate models for the time pre/post-listing and the time when the house is listed. It’s likely Realtor.com does as well which is why it stopped updating as soon as the house was listed.
• The on-the-market model, used during the listing period, are updated more frequently, this is probably due to the fact that they have more high-frequency data (views, time on market, etc.) and the possibility of making a commission on the property increases the amount of money they’re willing to spend running the models.
• Zillow and Redfin are both using the same data source to determine when to switch their models, and it appears to be different than the source they use to display if a home is actually listed or not. I don’t know why that would be. Possibly their models require data that is not immediately available?
• Zillow and Realtor.com require the same data for their model during the listing period and fail if that source is unavailable. Redfin continues estimating, but does so poorly.

1. The error message:

Where’s the Zestimate?

County transactional data for this home is insufficient so we cannot calculate a Zestimate. We are adding data all the time, so be sure to come back.