 When trying to predict "Y", the most naïve regression function that we can think of is the constant function predicting the mean of "Y", i.e., formula_8.It follows that the MSE of this function equals the variance of "Y"; that is, "SS"err = "SS"tot, and "SS"reg = 0.In this case, no variation in "Y" can be accounted for, and the FVU then has its maximum value of 1.