Phuong is conducting an experiment on proofreading. all participants are asked to read a passage with several typos and correct the errors. however, one group proofreads by reading silently, a second group proofreads by reading aloud, and a third group proofreads by reading the passage silently backward. the researcher calculates the average number of errors found in each group and finds that both the reading silently and reading aloud groups found more errors than the reading silently backward group. what are the variables in this study?