In previous chapters, we covered correlation and linear regression modeling in detail. If you look to quantify the relationship between two variables, you use the correlation coefficient. For example, you can quantify the relation between salary and expenses using correlation. If you needed to predict a response variable based upon some other item, you could use linear regression modeling, provided the relationship is linear. For example, if you want to predict exactly what a person’s expenses will be when his salary is $10,000, you can use linear regression modeling, provided the expense and salary fit on a straight-line graph. In some cases, this relationship is not actually linear, but you can make it linear by applying some simple mathematical transformations; still, you can use linear regression modeling.