Belfast Export$-$Import Partners has a large staff of buyers and sales personnel who travel extensively on company business. The CFO is trying to manage travel costs for the sales staff and has collected monthly information from the past $24$ months on sales dollars $($the dependent variable$)$ and travel costs $($the independent variable$).$ The regression results indicated the following relation: The regression results indicated the following relation: These results seemed to imply that travel costs were reducing sales. The CFO concluded that there must be a problem in the way the data were collected. Required Help the CFO. What might cause the negative relationship between travel costs and sales?