# Relationship between RMSE and RSS

I'm working on simple linear regression, and I would like to understand the relationship between RMSE and RSS (residual sum of squares).

In another Stackexchange question, I found some explanations, but they didn't directly explain the answer to my particular question, and definitely not in a way I could understand.

What is the relationship between RMSE and RSS in linear regression?

• RMSE = RSS/sqrt(# of residuals), I presume. – KH Kim Apr 9 '16 at 1:06

Having the mathematical derivations, you might ask yourself why use one measure over the other to assess the performance of a given model? You could use either, but the advantage of RMSE is that it will come out in more interpretable units. For example, if you were building a model that used house features to predict house prices, RSS would come out in dollars squared and would be a really huge number. RMSE would come out in dollars and its magnitude would make more sense given the range of your house price predictions.

• The RSS is the sum of the square of the errors (difference between calculation and measurement, or estimated and real values):

$$RSS = \sum{(\hat Y_i-Y_i)^2}$$

• The MSE is the mean of that sum of the square of the errors:

$$MSE = \frac{1}{n}\sum{(\hat Y_i-Y_i)^2}$$

• The RMSE is the square root of the MSE:

$$RMSE = \sqrt{MSE}$$

A bit of math shows:

$$RMSE = \sqrt{MSE} = \sqrt{\frac{1}{n} \cdot RSS}$$

You can check it in the example that you posted:

$$RMSE = \sqrt{\frac{1}{32} \cdot 447.6743} = 3.740297$$

Note that for the mtcars dataset $$n=32$$.

Also see this question

• Wait wait wait, then why is MSE not called MRS (Mean of Residual Squared)??? – NoName Mar 24 at 22:43
• @NoName It was like that when I got here! ¯_(ツ)_/¯ – Luis Mar 25 at 15:59