A study is conducted to determine whether people learn better with spaced or massed practice. Subjects volunteer from an introductory psychology class. The first 10 subjects who volunteer are assigned to the massed-practice condition; the next 10 are assigned to the spaced-practice condition. Discuss the consequences and seriousness of each of the following 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.
In general, which type of non-random sampling is more serious and why?