Problem:

Education and income. There is a strong positive association between workers' education and their income. For example, the U.S. Census Bureau reported in 2008 that the median income of young adults (ages 25 to 34) who worked full-time increased from $20,260 for those with less than a ninth-grade education, to $30,543 for high school graduates, to $46,932 for holders of a bachelor's degree, and on up for yet more education. In part, this association reflects causation-education helps people qualify for better jobs.

Required:

1. Suggest several lurking variables that also contribute. (Ask yourself what kinds of people tend to get more education.)