It is useful to consider the second definition to understand FVU. When trying to predict Y, the most naïve regression function that we can think of is the constant function predicting the mean of Y
. It follows that the MSE of this function equals the variance of Y
; that is, SSerr
, and SSreg
= 0. In this case, no variation in Y can be accounted for, and the FVU then has its maximum value of 1.