In this experiment design, the missing element is the control group.
In most experiments, the researcher will need to have a control group and an experimental group.
The control group is the one that is not given a treatment, for example, if you are testing an antibiotic, then the control group is the individuals that do not receive the medicine.
This group is important because it acts as a reference to measure the real effect in the experimental group or those receiving the treatment.
The control group is missing in this experiment because the research will need to measure the hours students spend studying on a regular cloudy day, in this way, he can later determine if there is a real difference between the number of hours on a sunny day vs the number of hours on a day that is not sunny.
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