This article reports an online poll conducted by CNN on people’s confidence in U.S. disaster preparedness.   It reports that 80% of the respondents did not have confidence in U.S. disaster preparedness. 

Questions:

a.  Read the first three paragraphs carefully.   What do you think of the sampling techniques used in this poll?  To what population can we generalize the results from this poll? 

b.  Your friend comments, “wow!  375,000 people responded to this survey!  We can definitely generalize this to the American population.”    What do you say? 

 

Schoolchildren practicing a "duck and cover" drill, designed to protect them from a nuclear attack.  (Photo: Bettmann/Corbis)

 

Suggested answers:

 a.  Notice that the poll was presented on the CNN website, which means that its sample was almost certainly self-selected.  (Another clue? A truly representative survey would never have called a random sample of 375,000 people—2,000 randomly selected people would have been plenty–see Table 6.1.) The upshot is that we have no idea to whom we can generalize the results, because we don’t know the characteristics of the type of people who choose to respond to this online poll.  They are cnn.com readers (who may be different from non-readers), and they also may have chosen to respond to the poll because they have a strong negative or positive opinion about the issue.

 b. Your friend is wrong; the number of participants in a sample has virtually nothing to do with the sample’s ability to generalize to a population.  What matters is not how many, but how.  If the sample is a probability sample, we can generalize to its population.  If not, we cannot.