Study Note: Dimension Reduction - PCA, PCR
Dimension Reduction Methods Subset selection and shrinkage methods all use the original predictors, X1,X2, . . . , Xp. Dimension Reduction Methods transform the predictors and then fit a least squares model using the transformed variables. Approach Let $Z_1,Z_2, . . . ,Z_M$ represent $M < p$ linear combinations of our original $p$ predictors. That is, $$ \begin{align} Z_m=\sum_{j=1}^p\phi_{jm}X_j \end{align} $$ ...