Variables are “qualities” (or less technically, “things”) that we measure, manipulate, and control in research and data analysis. Variables are typically represented by sets (vectors) of values, which in turn represent measurements assigned to observations on a particular scale of measurement (e.g., nominal, ordinal, interval, or ratio scales), following the rules of measurement that characterize the specific variable. For example, the variable Gender can be measured on a nominal scale (Male, Female); the variable Weight can be measured on a ratio scale (165 lb., 190 lb., 229 lb., etc.).