Nora is interested in differences in amount of content retained from a lecture based on time of class. She compares her two sections--one that meets at 8 a.m. and one that meets at 2 p.m.-- by testing them, unannounced, on content that she taught two weeks ago.

1. Which experimental design is Nora best illustrating? In your answer, include the notation (R, O, X etc)