Researchers were interested in the effect of pre-existing inappropriate highlighting of text on reading comprehension. They randomly assigned a group of 600 students to two groups. Both groups were asked to answer a reading comprehension test. The text given to the first group had inappropriate passages highlighted, while the text of the second group wasn’t highlighted at all. Then, the researchers compared the average score of each group in the reading comprehension test. What type of statistical study did the researchers use?