Before beginning a unit on frog anatomy, a seventh-grade biology teacher gives each of the 24 students in the class a pretest to assess their knowledge of frog anatomy. The teacher wants to compare the effectiveness of an instructional program in which students physically dissect frogs with the effectiveness of a different program in which students use computer software that only simulates the dissection of a frog. After completing one of the two programs, students will be given a posttest to assess their knowledge of frog anatomy. The teacher will then analyze the changes in the test scores (score on posttest minus score on pretest). Required:
a. Describe a method for assigning 24 students to two groups of equal size that allows for a statistically valid comparison of the two instructional programs. Suppose the teacher decided to allow in the class to select which instructional program on frog anatomy (physical dissection or computer simulation) they prefer to take, and 11 students choose actual dissection and 13 students choose computer simulation.
b. How might that self-selection process jeopardize a statistically valid comparison of the changes in test scores (score on posttest minus score on pretest) for the two instructional programs?