Chapter 10

The Paired-Samples t Test

Holiday Weight Gain and Two-Group Studies Two-group studies indicate that the average holiday weight gain by college students is less than many people believe—only about 1 pound.

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In many parts of the world, the winter holiday season is a time when family food traditions take center stage. Popular wisdom suggests that during this season, many Americans put on 5 to 7 pounds. But before-and-after studies suggest a weight gain of just over 1 pound (Hull, Radley, Dinger, & Fields, 2006; Roberts & Mayer, 2000; Yanovski et al., 2000). A 1-pound weight gain over the holidays might not seem so bad, but weight gained over the holidays tends to stay (Yanovski et al., 2000).

The fact that researchers used two groups in their study—students before the holidays and students after the holidays—is important for this chapter. With a t distribution, we can compare one sample to a population when we don’t know all the details about the parameters, as we did in Chapter 9, and we can compare two samples to each other. There are two ways to compare two samples: by using a within-groups design (as when the same people are weighed before and after the holidays) or by using a between-groups design (as when different people are in the preholiday sample than those in the postholiday sample). For a within-groups design, we use a paired-samples t test. The steps for a paired-samples t test are similar to those for a single-sample t test, which we learned about in Chapter 9. (For a between-groups design, we use an independent-samples t test, which we will learn about in Chapter 11.)