An analyst became puzzled when analyzing the performance of franchises operated by a fast-food chain. The correlation between sales and the number of competitors within 3 miles was positive. When she...

An analyst became puzzled when analyzing the performance of franchises operated by a fast-food chain. The correlation between sales and the number of competitors within 3 miles was positive. When she regressed sales on the number of competitors and population density, however, she got a negative slope for the number of competitors. How do you explain this?