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., \(log(x), x^{2}\).