4. A random sample of 15 automobile mechanics certi ed to work on a certain type of car was selected, and the time (in minutes) necessary for each one to diagnose a particular problem was determined, resulting in the following data:

30.6 30.1 15.6 26.7 27.1 25.4 35.0 30.8 31.9 53.2 12.5 23.2 8.8 24.9 30.2 Use the Wilcoxon test at signi cance level .10 to decide whether the data suggests that true average diagnostic time is less than 30 minutes.