Sampling

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Modern public opinion polling depends critically on sampling for its viability, since it rests on the assumption that we can learn about what the public as a whole thinks by asking just a fraction of its members. To get a better feel for how this works, consider these questions:

First, what is sampling? As the video clip above suggests, sampling refers simply to the selection of a subset of individuals (i.e. a "sample") from a larger population. ABC News's "Guide to Polls and Public Opinion" points out that sampling isn't unique to public opinion polling; instead, it's analogous to a blood test, in which only a little bit of blood is studied in order to draw conclusions about the health of an entire organism. Another good analogy is to a chef who tastes just a spoonful of sauce to determine whether it's ready to serve. In both examples, it would be ludicrous to suggest that the entire bloodstream or pot of sauce would need to be tested; so too in public opinion polling, only a small sample of the population is needed to make determinations about the views and attitudes of the whole.

Second, what are the characteristics of a good sample? In a nutshell, good sampling accounts for two factors: size (how big the sample is) and representativeness (the extent to which the sample faithfully reflects key characteristics of the whole population). Of the two, representativeness matters more than size. Consider the contest between the Literary Digest and Gallup polls to predict the 1936 election (see here to refresh your memory). The Literary Digest poll had a significantly larger sample of millions of respondents compared to Gallup's couple of thousand. However, Gallup's quota sampling technique provided a sample that was more representative of the public as a whole than the Literary Digest's reliance on telephone and automobile registry listings, which tended to oversample voters from the wealthier end of the socioeconomic spectrum. That said, size does matter, especially when it comes to determining a poll's margin of error, as suggested by another one of our required texts for this unit: Public Agenda's "Best Estimates: A Guide to Sample Size and Margin of Error."

Finally, how should samples be selected? The final required text for this unit, "Types of Sampling" from the Columbia Center for New Media Teaching and Learning, lays out several types of sampling strategies that are commonly used in public opinion polling and the circumstances that are best suited to the different techniques.