https://cran.r-project.org/web/packages/corrplot/vignettes/corrplot-intro.html

https://rpubs.com/toleafar/210620

https://www.r-bloggers.com/using-apply-sapply-lapply-in-r/

1. (a) 

To test the quality of a regression fit use mean squared error.

       1   ₙ
MSE = ───  ∑ (yᵢ - f̂(xᵢ))² 
       n   ⁱ⁼¹



Ave(y₀ - f(x₀))²


To test the quality of a model in the classification settings, use the error rate:

         1   ₙ
    E = ─── ∑ I(yᵢ ≠ ŷᵢ)
         n   ⁱ⁼¹
