What are the observational units?

Can people better memorize letters if they are presented in recognizable groupings than if they are not? Students in an introductory statistics class were the subjects in a study that investigated this question. These students were given a sequence of 30 letters to memorize in 20 seconds. Some students (25 of them) were given the letters in recognizable three-letter groupings such as JFK-CIA-FBI-USA-…. The other students (26 of them) were given the exact same letters in the same order, but the groupings varied in size and did not include recognizable chunks, such as JFKC-IAF-BIU- … The instructor decided which students received which grouping by random assignment. After 20 seconds of studying the letters, students recorded as many letters as they could remember. Their score was the number of letters memorized correctly before their first mistake. The instructor conjectured that students in the JFK-CIA-… group would memorize more letters, on average, than students in the JFKC-IAF-… group.