Biology 2244A/B Lecture Notes - Lecture 2: Confounding, Nominal Level, Ratio Test
22 Jan 2015
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A voluntary response sample (or self-selected sample) is one in which the respondents themselves decide whether to be included. People who participate often do so because of interests and opinion, so the responses are not representative of the whole population. This suggests that mail-in or fax surveys, internet surveys, and telephone call-in polls are flawed in the sense that we should not make conclusions based on such biased samples. You could only conclude about the specific group of people who chose to participate. If sample data were not collected in an appropriate way, the data may be completely useless. We obtain data from two sources: observational studies and experiments. In an observational study, we observe and measure specific characteristics, but we don"t attempt to modify the subjects being studied. Gallup poll (which is observational in the sense that we merely observe people, often through interviews, without modifying them in any way).
