这里的RSS为什么直接平方加起来就可以

问题如下：

A given set of data was divided into three equal parts. Three separate models were developed—each model using two of the three parts for fitting. Errors were calculated for each model. The diagram below shows the standard errors for each model run (the orange highlights where the data was used for fitting the model versus the blue indicating the data that was held back):

Using the principles of m-fold cross validation, which model should be selected?

选项：

A.

M1

B.

M2

C.

M3

解释：

The first task is to calculate the squared residuals:

The model selected is the one that has the smallest RSS within the blue out-of-sample boxes—this is M3.