Over the past 50 years, there has been a strong negative correlation between average annual income and the record time to run 1 mile. In other words, average annual incomes have been rising while the record time to run 1 mile has been decreasing.
(a) Do you think increasing incomes cause decreasing times to run the mile? Explain.
(b) What lurking variables might be causing the change in one or both of the variables? Explain.