Students who smoke (x) earn lower grades (y) than students who do not smoke. conclusion: smoking causes poor grades.

A.Students who smoke (x) earn lower grades (y) than students who do not smoke. conclusion: smoking causes poor grades.

B. Expectant mothers who drink bottled water (x) have healthier babies (y) than do expectant mothers who do not drink bottled water. Conclusion: drinking bottled water causes healthier babies to be born.

C. Tea Drinkers (x) are more likely to be democrats (y) than are non-tea drinkers. conclusion: drinking tea causes people to be democrats

Question is… Think up a plausible variable, z, that defines a compositional difference across the values of x. describe how Z creates the relationship between X and Y.