A study is conducted to determine whether people learn better with spaced or massed practice. Subjects volunteer from an introductory psychology class. At the beginning of the semester 12 subjects volunteer and are assigned to the massed-practice condition. At the end of the semester 12 subjects volunteer and are assigned to the spaced-practice condition. This experiment involves two kinds of non-random sampling: (1) Subjects are not randomly sampled from some specified population and (2) subjects are not randomly assigned to conditions. Which problem is more serious? Question 3 options:

Random sampling is more important than random assignment because there could be systematic differences between students in the class and those from the real world.

Random assignment is more important than random sampling because there could be systematic differences between students in the class and those from the real world.

Random assignment is more important than random sampling because there could be systematic differences between students who volunteer at the beginning of the semester and those who volunteer at the end.

Random sampling is more important than random assignment because there could be systematic differences between students who volunteer at the beginning of the semester and those who volunteer at the end.