Suppose a simple random sample of college freshmen are selected from two universities - 15 students from Xavier University (XU) and 20 students from the University of Cincinnati (UC). On a standardized test, the sample from XU has an average score of 1000 with a standard deviation of 24, and the sample from UC has an average score of 950 with a standard deviation of 18. Construct a 99% confidence interval for the difference in these scores.