28. A physics instructor believes that natural lighting in classrooms improves student learning. He conducts an experiment in which he teaches the same physics unit to two groups of seven randomly assigned students in each group. Everything is similar for the groups, except that one of the groups receives the instruction in a classroom that admits a lot of natural light in addition to the fl uorescent lighting, while the other uses a classroom with only fl uorescent lighting. At the end of the unit, both groups are given the same end-of-unit exam. There are 20 possible points on the exam; the higher the score, the better the performance. The following scores are obtained.

Natural Plus Fluorescent Lighting Fluorescent Lighting Only 17 16 18 13 14 12 17 14 16 13 19 17 17 15 14