StatCrunch logo (home)

Report Properties

from Flickr
Owner: hdoman78
Created: Feb 28, 2013
Share: yes
Views: 1673
Results in this report
Data sets in this report
Need help?
To copy selected text, right click to Copy or choose the Copy option under your browser's Edit menu. Text copied in this manner can be pasted directly into most documents with formatting maintained.
To copy selected graphs, right click on the graph to Copy. When pasting into a document, make sure to paste the graph content rather than a link to the graph. For example, to paste in MS Word choose Edit > Paste Special, and select the Device Independent Bitmap option.
You can now also Mail results and reports. The email may contain a simple link to the StatCrunch site or the complete output with data and graphics attached. In addition to being a great way to deliver output to someone else, this is also a great way to save your own hard copy. To try it out, simply click on the Mail link.
Zestimate vs Sale Price Tucson, AZ-H.Domanski
Mail   Print   Twitter   Facebook

The housing market can be a volatile place; multiple variable impact the sale price and final sale price.  Zillow has come up with a way to account for the market variability in their "Zestimate" price.  A direct application of Zestimate vs. Actual Sale Price for the period from January 23 to February 23, 2013 in Tucson, Arizona is shown in the statistical data.  With 13 sales for the aforementioned period, a few factors indicating a positive linear relationship emerge:

1.- The critical value  is .576 for the sample size of 13.  The sample correlation co-efficient is .896, which is greater than the critical value.

2.-The scatter plot has no discernable pattern when Zestimate residuals are plotted.

3.-When viewing the scatter plot and least squares regression line, there is a discernable pattern of positive correlation between the Zestimate and the Actual Sales Price. That is to say, for every $1 of Zestimate value, the Actual Sales Price goes up .963.

4.-The co-efficient of determination is .802 which is close enough to one to indicate that the line of regression does a good job of explaining the variation of the response variable.

There is no need to interpret the y-intercept since the value of x (price) will never be zero.  Would that it were true!  A free home!

Upon assembly of the box plot however, there are extreme variables that emerge that indicate the case for linear relationship may not be so air-tight. Upon investigation of these two outliers, the following was found:

1.-The Zestimate that overstated a possible sales price (selling for $70,000 when the Zestimate was $106,477) was on a home in danger of a "short sale" and was marketed to be a sold quickly.

2.-The Zestimate that overstated a possible sales price (selling for $120,000 when the Zestimate was $176,706) was on a home also in danger of a "short sale".

These outliers would seem to indicate that the Zestimate is not a viable tool, when perhaps the reason for the unexplained variability is related to lurking variables such as a short sale when the house is sold far below market value.  Instead, it would be fair to say that a Zestimate for a house when all of the available variables (such as area values, other sales of comperable properties, condition of home) are taken into account is an accurate tool; keeping in mind that when people are put into the mix anything can happen.

Overall, a linear model does work for this set of values (and probably for any market for Zestimate vs. Actual Sales price) when the prices are tempered with knowledge of the circumstances surrounding the sale of the house (a standard sale vs a short sale).


Result 1: Simple Linear Regression for Zestimate vs Actual Sales Price   [Info]
Simple linear regression results:
Dependent Variable: Selling Price
Independent Variable: Zestimate
Selling Price = -2746.139 + 0.96284103 Zestimate
Sample size: 13
R (correlation coefficient) = 0.896
R-sq = 0.8028648
Estimate of error standard deviation: 21291.787

Parameter estimates:
Parameter Estimate Std. Err. Alternative DF T-Stat P-Value
Intercept -2746.139 16398.111 ≠ 0 11 -0.16746677 0.87
Slope 0.96284103 0.14385307 ≠ 0 11 6.6932254 <0.0001

Analysis of variance table for regression model:
Source DF SS MS F-stat P-value
Model 1 2.03093094E10 2.03093094E10 44.799267 <0.0001
Error 11 4.9867423E9 4.53340224E8
Total 12 2.52960522E10

Result 2: Scatter Plot Zestimate vs Actual Sale price   [Info]
Right click to copy

Result 3: Boxplot Residuals for Zestimate   [Info]
Right click to copy

Result 4: Scatter Plot Zestimate vs Residuals   [Info]
Right click to copy

Data set 1. Zestimate 2013   [Info]
To analyze this data, please sign in.

HTML link:
<A href="">Zestimate vs Sale Price Tucson, AZ-H.Domanski</A>

Want to comment? Subscribe
Already a member? Sign in.
By msullivan13803
Mar 6, 2013

Nicely done.
By jspaintme
Mar 2, 2013

Julie Stengel r/t previous post
By jspaintme
Mar 2, 2013

I really like your report. I also agree with skhati45's point.
There is so much to take into account when estimating home prices. I agree that lurking variables may definitely be a factor in the resulting outliers.
By skhati45
Feb 27, 2013

Very nice! Straight to the point! I think it was a good idea to use statistics from because a lot of people have actually heard of this website and might have an easier time interpreting results.

~ Summer Khatib

Always Learning