Theory: a systematic way of organizing & explaining observations. Provides the framework for generating a hypothesis: proposed cause-and-effect relationship between 2 or more variables, that flows from the theory or an important question.
Variable: a phenomenon that changes across circumstances or varies among individuals. Can be-
continuous variable: can be placed on a continuum of infinite values (intelligence, body weight)
categorical variable: can take on fixed values (sex- male or female, race, had heart attack-yes or no)