## 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., $$log(x), x^{2}$$.