One way to help students learn new quantitative properties (as opposed to seeing
the ones they already know) is to use contrasting cases of small data sets. For
example, Figure 1 provides an activity used to prepare students to learn about the
mean deviation formula. Each grid shows the result of a test using a different baseball-pitching
machine. The black circles represent where a pitch landed when
aimed at the target X. Students work in small groups to develop a reliability index.
Their task is to develop a formula or procedure that computes a single value to help
shoppers compare reliability between the machines, much as an appliance receives
an efficiency index. The contrasts between the four machines draw attention to issues
of sample size, outliers, density, and the distinctions between variability, central
tendency, and accuracy