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Imagine that an honours thesis student wanted to

compare students who participated in a weekly study group for statistics, to

students who did not participate in the weekly study group.  She found 40 statistics students willing to

participate in her study, and randomly assigned half to participate in the

study group while the rest did not. 

Then, she compared their final grades at the end of the semester to see

whether those who participated in the study group earned higher final grades.

In this example, the independent variable