Root mean squared error

Root mean squared error, also known as RMSE, is arguably the easiest to understand given the previous methods. Take the following plot:

In the case of testing the model as we did previously with the holdout set, the red dots are the actual values from the test set, while the blue dots are the predicted values. The X depicted is the distance between the predicted and actual values. RMSE simply takes a mean of all of those distances, squares that value, and then takes the square root. 

A value under 180 is generally considered a good model.