A student researcher conducts an experiment that investigates the effects of a new textbook on student achievement. Two intact classrooms are used for the samples. One classroom is taught with the new textbook and the other is taught with the traditional textbook. Each sample consists of 30 students. Observations of student engagement are rated by the researcher, according to an engagement scale. How could this study be improved? Select all that apply A. Have another researcher rate the observations, as well. B. Administer an instrument, gauging student understanding. C. Randomly assign students to two different groups. D. Restrict the study to one group- the one taught with the new textbook
