13. The file pyge.dat (see Section 1.8) contains pretest reasoning IQ scores for students in grades 1 and 2 who were assigned to one of three ability tracks. (The data are from Elashoff and Snow, 1970, and were originally collected by R. Rosenthal.) The file pygc.dat contain data for a control group. The experimental group consisted of children for whom positive expectancies had been suggested to teachers. Compare the 20%

trimmed means of the control group to the experimental group using the function yuen, and verify that the .95 confidence interval is (−7.12, 27.96). Thus, you would not reject the hypothesis of equal trimmed means. What reasons can be given for not concluding that the two groups have comparable IQ scores?