Sampling on campus. You see a woman student standing in front of the student center, now and then stopping other students to ask them questions. She says that she is collectiong student opinions for a class assignment. Explain why this sampling method is almost certainly biased.

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Actually there's several reasons. The main idea is that it's not a truly random sample. This researcher is limiting herself to students who visit the student center and do so at that particular time of day. What about students who don't visit the student center? Or those who do so at alternate times of day? Add to that the fact that by selectively questioning certain individuals she's handpicking her subjects. Recall the definition of "random" ; picking your sample in such a way that every member of the population in question has an equal probability of being selected.