In an experiment on braking performance, a tire manufacturer measured the stopping distance for one of its tire models. On a test track, a car made repeated stops from 60 miles per hour. Twenty tests were run, 10 each on both dry and wet pavement, with results shown in the table. (Note that actual braking distance, which takes into account the driver’s reaction time, is much longer, typically nearly 300 feet at 60 mph!)
Stopping Distance (ft)
Dry Pavement Wet Payment
145 ............ 211
152 ............ 191
141 ............ 220
143 ............ 207
131 ............ 198
148 ............ 208
126 ............ 206
140 ............ 177
135 ............ 183
133 ............ 223
a) Find a 95% confidence interval for the mean dry pavement stopping distance. Be sure to check the appropriate assumptions and conditions, and explain what your interval means.
b) Find a 95% confidence interval for the mean increase in stopping distance on wet pavement. Be sure to check the appropriate assumptions and conditions, and explain what your interval means.