Identify the Procedures in the Following Research Description That Are

Identify the procedures in the following research description that are likely to cause sampling bias: A researcher teaches three sections of a remedial English class for freshmen in a large state university. A colleague teaches three sections of regular freshman English. The researcher has developed a special 20-hour program to teach rules of spelling. She gives a spelling test in her sections and in her colleague's sections. She identifies 63 students from her sections and 36 from her colleague's sections who scored below 50 percent on the test. She then describes the special program to her sections and asks the 63 low scorers to take the program by attending special sessions one hour per day for four weeks. Fifty-one agree to do so. The 36 students in her colleague's sections are used as a control group and receive no special treatment. At the end of four weeks, 26 of her students have completed the spelling program, while the remainder have missed between 1 and 18 of the sessions. She administers a spelling posttest to the 26 treatment participants and the 36 control participants and compares their gains since the pretest.