Leading up to the winter of 2004-2005, there was a shortage of flu vaccine in the United States due to impurities found in the supplies of one major vaccine supplier. The Harris Poll took a survey to determine the effects of that shortage and posted the results on the Harris Poll Web site. Following the posted results were two paragraphs concerning the methodology, of which the second one is shown here.
In theory, with probability samples of this size, one could say with 95 percent certainty that the results have a sampling error of plus or minus 2 percentage points. Sampling error for the various subsample results is higher and varies. Unfortunately, there are several other possible sources of error in all polls or surveys that are probably more serious than theoretical calculations of sampling error. They include refusals to be interviewed (non-response), question wording and question order, and weighting. It is impossible to quantify the errors that may result from these factors. This online sample is not a probability sample.
a. Note the last sentence. Why do you think that this sample is not a probability sample?
b. Is the sampling process any one of the other sampling designs discussed in this section: systematic random sampling, cluster sampling, stratified sampling, or multistage sampling? For each sampling design, explain your answer.