Aresearcher has conducted an experiment in which she assigned all of her students four hours of homework the night before a test, in order to see what effect the extra stress has on test performance. the study is flawed because the researcher did not include a(n)

The study is flawed because the researcher did not include a control group.

Explanation:

The control group is a necesary element in experimental science, it is the standard to which the researcher can compare the analysis and draw conclusions. A control group has the same characteristics as the experimental group, but it is not subject to treatment or manipulation of variables like the experimental group is. This way, the researcher can assess the differences in the dependent variable between both groups.

In this case, the researcher asigned all of her students the same amount of homework, when she measures the extra stress that it causes she will not be able to relate the extra homework with the stress. If she had a control group to which she asigned less homework, and measured the stress in both groups, she can observe how much more stressed the students in the experimental group are and relate it to the extra homework they received.