Dr. johansen randomly assigned research participants to three different groups during her last experiment. she then proceeded to give all the participants in the experiment a new study technique designed to enhance their learning for the upcoming test. what critical error did she make during her experiment?

She did not create a control group for this test. There wasn't a group that had no treatment (or study technique, in this case). By giving each group a new technique and a fourth group no technique at all before measuring their learning, Dr. Johansen will not be able to credibly say if any of the techniques increased the subjects' learning potential.