Exercise

# t-interval at various levels

A random sample was taken of nearly 10% of UCLA courses. The most expensive textbook for each course was identified, and its price at the UCLA Bookstore and on Amazon.com were recorded. These data are recorded in the `textbooks`

dataset. We want to test whether there is a difference between the average prices of textbooks sold in the bookstore vs. on Amazon.

Since the book price data are *paired* (the price of the same book at the two stores are not independent), rather than using individual variables for the prices from each store, you will look at the a single variables of the differences in price. The `diff`

column the UCLA Bookstore price minus the Amazon.com price for each book.

Instructions

**100 XP**

- Construct a 90% confidence interval for the average difference in prices of the same textbook at at the UCLA bookstore and on Amazon.
- Now construct the same interval at the 95% confidence level.
- And now at the 99% confidence level.
- Take a note of the widths of the intervals. You'll need these for the next exercise.