A Music Professor Offers His 40 Students the Option of Coming

Multiple Choice

A music professor offers his 40 students the option of coming to an additional rehearsal session the week before their juries (musical final exams.) In order to decide whether these extra sessions actually help students, he keeps track of who attends them and compares their jury scores to those of students who did not schedule extra sessions.
This study is a(n) :

A) observational study.
B) nonrandomized experiment.
C) completely randomized experiment.
D) randomized block design.
E) matched pairs design.