2.4 Numerical Measures of Variability

Measures of central tendency provide only a partial description of a quantitative data set. The description is incomplete without a measure of the variability, or spread, of the data set. Knowledge of the data set’s variability, along with knowledge of its center, can help us visualize the shape of the data set as well as its extreme values.

For example, suppose we want to compare response time to a stimulus for subjects treated with two different drugs, A and B. The histograms for the response times (in seconds) for each of the two drugs are shown in Figure 2.19. If you examine the two histograms, you’ll notice that both data sets are symmetric, with equal modes, medians, and means. However, Drug A (Figure ...

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