The accompanying data file contains information on 100 working adults’ self-assessed happiness on a scale of 0 to 100, along with their age and annual income. Consider two quadratic models for happiness. Model 1 is based on age, age², and income. Model 2 is based on age, age², and ln(income). 

a. Use the holdout method to compare the predictability of the models using the first 75 observations for training and the remaining 25 observations for validation. 

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