Tiffany is interested in the effects of different kinds of instructions on exam performance. she asks 60 college freshmen to study regularly for an upcoming exam, which they will have 1 hour to take. participants are randomly assigned to one of three instruction groups: (1) complete the exam as quickly as possible, (2) complete the multiple choice before the short answers and essays, or (3) complete the short answers and essays before the multiple choice. obviously, even in a single instruction group, not all students will obtain the same exam score. these differences in an instruction group reflect: