A professor is teaching a large class that has 247 students. He wants to select a sample of 15 students to do a study on the habits of his students. For each of the following sampling methods, explain what kind of method is used (e.g., is it a random sample, nonrandom sample, sample with replacement, sample without replacement, convenience sample, judgment sample, quota sample).
a. There are 15 sociology majors in his class. He selects these 15 students to include in the sample.
b. He goes through the roster and finds that he knows 30 of the 247 students. Using his knowledge about these students, he selects 15 students from these 30 to include in the sample.
c. He enters names of all 247 students in a spreadsheet on his computer. He uses a statistical software (such as Minitab) to select 15 students.