Opinion Polls

Methodological Issues

The notion of representative sampling is one of the methodological pivots of opinion polls. A sample is a group of individuals who are selected as representative of a given population (citizens, voters, consumers, youngsters, housewives, etc.) through random extraction (probability sampling). According to the [Page 1059]mathematical theory of probability, assigning each member of a population with an equal probability of being selected is a sufficient condition for obtaining a final sample that is relatively similar to the whole population, with a predictable degree of approximation, which depends on the sample size. Due to an inherent statistical bias known as sampling error, the expected gap between the values in the sample and in the population (or the confidence interval) is, for example, around 3 percentage points for a sample of one thousand interviewees—the standard size in contemporary opinion polling. This means that an opinion expressed by, say, 28 percent of interviewees more realistically reflects the opinion of a proportion of between 25 and 31 percent (28 percent plus/minus 3 percentage points) of the observed population. In addition to probability sampling, another technique, known as quota sampling, is used, particularly by non–Anglo-American commercial pollsters, to ensure certain sociodemographic variables (sex, age, region, occupation, etc.) with exactly the same proportion in the sample as in the general population. Although it may be effective for practical purposes, the quota sampling technique (or nonprobability sampling) is regarded as arbitrary from a statistical theory perspective, since it is incongruent with the law of probability. However, since the strict theoretical assumptions required for probability sampling are systematically violated in real-world practices (i.e., low response rates among the originally sampled individuals), a mixed method combining probability and quota sampling can be considered as standard procedure in modern public opinion polling.

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