Well, this seems like a nice random sample that could be nicely representative of the entire population. Ask all students whose names begin with N or B. Well, this could be perceived as kind of random, but notice N is the same starting letter as Napoleon. B is the same starting letter as Blair. You might say, well, that's fair. You're doing it for each of their letters, but maybe there's like 10 people whose names start with an N and only two people whose names start with a B. Once again, you're not even getting a large sample.