A test prep course takes a random sample of 50 of its customers. Researchers score each customer on a diagnostic test before taking the course and again on a similar test after taking the course. They want to use these results to estimate the average difference between the before and after scores.
What type of statical test used in this case?
A. A t-test for a mean
B. A z-test for a proportion
C. A paired t-interval for the mean difference
D. A z-interval for a proportion
E. A t-interval for a mean
