8.1 Types of X variables

• Fixed: The levels of \(X\) are selected in advance with the intent to measure the affect on an outcome \(Y\).
• Variable: Random sample of individuals from the population is taken and \(X\) and \(Y\) are measured on each individual.
• X’s can be continuous or discrete (categorical)
• X’s can be transformations of other X’s, e.g., \(log(x), x^{2}\).