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Two or more surveys taken a number of years apart may show a trend in opinion or at least a major change. However, such a change may be due to a shift in opinion among the members of a population, a change in the composition of the population itself, or a combination of these two types of change. An attempt to disentangle the two types of change is presented in the following article.
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William M. Evan (1959) studied this question.
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