1. Multivariate Relationships
Goal: Show a causal relationship between two variables (X  Y)
Elements of a cause-and-effect relationship:
Association between variables (based on methods we’ve covered this semester)
Correct time order (X occurs before Y)
Elimination of alternative explanations (variable Z that acts on both X and Y, making them appear to be associated)
Anecdotal evidence does not rule out causality

2. Controlling for Other Variables
Observational studies: Researchers are unable to control levels of variables and may only observe them as they occur in nature
Statistical Control: Identifying individuals (cases) by their level of an alternative explanatory (control) variable (although not assigning subjects the levels)
Spurious Association: When both variables of interest are dependent on a third variable, and their association vanishes when controlling for the other variable X1 X2 Y