Refer to Exercise 9.39.

For these counties, the correlation between high school education rate and median income equals 0.79.

Suppose we also have data at the individual level as well as aggregated for a county. Sketch a scatterplot to show that at the individual level, the correlation could be much weaker. (Hint: Show that lots of variability could exist for individuals, yet the summary values for counties could fall close to a straight line. Thus, it is misleading to extend results from the aggregate level to individuals. Making predictions about individuals based on the behavior of aggregate groups is known as the ecological fallacy.)