A certain test preparation course is designed to help students improve their scores on the LSAT exam. A mock exam is given at the beginning and end of the course to determine the effectiveness of the course. The following measurements are the net change in 6 students' scores on the exam after completing the course: 18, 19, 13, 7, 13, 13 Using these data, construct a 99 % confidence interval for the average net change in a student's score after completing the course. Assume the population is approximately normal. Copy Data Step 1 of 4: Calculate the sample mean for the given sample data. Round your answer to one decimal place