Which of the following examples show a correlation between two variables that is masked by grouping over a third variable.

Variables: number of hours spent studying and number of credit hours enrolled
Populations: college students at a community college and college students at a neighboring community college

Variables: hours spent at the gym and weight
Populations: professional football players and Olympic track runners


Variables: weight of vehicle and average miles per gallon
Populations: large school buses and short school buses

Variables: a person's income and the square footage of their apartment
Populations: adults living in an rural mountain town and adults living in an rural farm town

Variables: miles driven and speeding tickets
Populations: drivers of blue cars and drivers of green cars