2. How Values Cluster Together

When you think about a group that’s measured on some numeric variable, you often start thinking about the group’s average value. On a scale of 1 to 10, how well do registered Independents think the president is doing? What is the average market value of a house in Minneapolis? What’s the most popular first name for boys born last year?

The answer to each of those questions, and questions like them, is usually expressed as an average, although the word average in everyday usage isn’t well defined, and you would go about figuring each average differently. For example, to investigate presidential ...

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