We do this test this way because we need to control the amount of computer time each group has, or we won't have an experiment.
Since we want to see if there is causality between high computer usage and high blood pressure, we need then one group, the control, to have a low computer usage, and the experimental group, what we're testing for, to have higher computer time usage. So the question for the experiment is: "does high computer usage time cause high blood pressure (is there causality)?" You can think of the reason for the experiment is that, in the observational study, we saw a positive correlation between computer usage and blood pressure. With this experiment example, Sal is trying to see if the amount of computer time has an impact on blood pressure.
Here's what I got from this video, and I hope this helps: what is done with the control group and the experimental group will vary on what you're testing for.
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