A police agency is interested in reducing the speed of cars on a particular stretch of highway. Lore has it that a visible police car with radar is more effective in reducing speed than a warning sign saying that the area is monitored by radar. A police sergeant wonders if the lore is correct, so he tests each method on the same stretch of highway at the same time of day and on the same day of the week (two consecutive weeks) to ensure similar driving volume and conditions. He then records the number of miles per hour (mph) each speeder drives over the speed limit. Here are the data of the excess speed of the eight drivers caught speeding under each condition. Conduct a one-tailed t test at the .05 a level. Does the police car result in lower speeds than the warning sign?
Excess mph With Police CarExcess mph With Warning Sign
12........................................18
7........................................12
6........................................ 9
10........................................15
4........................................ 7
13........................................19
11........................................18
6........................................12