The accompanying data file contains survey information on 120 American adults who rated their health (Health) and social connections (Social) on a scale of 1 to 100. The file also contains information on their household income (Income, in $1,000s) and college education (College equals 1 if they have completed a bachelor’s degree, 0 otherwise). Consider two linear regression models for Health. Model 1 uses Social, Income, and College as the predictor variables, whereas Model 2 also includes the interactions of Social with Income and Social with College. 

a. Use the holdout method to compare the predictability of the models using the first 90 observations for training and the remaining 30 observations for validation. 

b. Use the k-fold cross-validation method to compare the predictability of the models using k = 4.