Categories and Population Proportions
Chapters 7, 8, and 9 dealt with making inferences about a population on the basis of a sample when the observation of interest was a measurement whose mean in the population we wished to know. In Chapter 6 we discussed a different kind of observation, one based on categories rather than measurements. If the observation of interest involves a set of categories rather than a measurement, it of course makes no sense to think in terms of the center of a batch or its spread. Rather, we approach the batch in terms of proportions. When we observe categories in a sample, then, our basic thought about the population from which the sample was selected concerns the proportions of the different categories in the population, not the mean of anything.
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