This study is a quasi-experiment. At first glance, this study might appear to be an experiment but is lacking the essential component of random assignment. Students selected their night of testing, thus the study is a quasi-experiment. The independent variable was the amount of caffeine, and the dependent variable was the length of time to fall asleep (sleep latency). As it is a quasi-experiment, the researchers should not have made a statement implying that amount of caffeine impacts sleep latency. There are other differences between the two groups of students besides the independent variable. For example, the group who chose to participate on Tuesday night may all have a Wednesday night class with an exam coming up, thus may be more stressed and have greater difficulty sleeping. Thus, its not the regular coffee but the stress that is impacting sleep latency. Also, the study was conducted over two separate days. The day may be responsible for the difference observed and not the type of coffee. For example, it might have been storming on Tuesday, thus making it more difficult to fall asleep.
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