Unit 3, Inventing Precision, employs what students have learned from Units 1 and 2 to focus on the variability of their data. Students invent a measure of precision. Precision refers to the tendency of the measurements to agree, to be consistent from measurer to measurer. Students use a software tool, TinkerPlots, to develop this measure (a statistic of variability). Some student inventions use the distance between each measurement and the center, that is, a deviation of each value from the median or mean, to summarize the tendency for the measurements to agree. This solution is an analog to the mean deviation statistic. Other inventions tend to focus on the boundaries of the center clump, an analog to the inter-quartile range. Yet other inventions focus on the extreme scores—the range statistic. Most student solutions are unconventional, but contain the seeds of the mathematical thinking that motivates more conventional approaches. Once again, students compare their methods with those of others, focusing on what aspects of the data the inventor employed to generate the measure. With teacher guidance, the class considers the quality of the measure proposed: Does it provide a way to describe the tendency of the measurements to agree? What happens to the statistic if some of the measurements change? Students’ invented statistics are compared to three conventional statistics: range, mean absolute deviation, and inter-quartile range. Often, teachers ask students to compare the precision of measure obtained with the 15 cm. ruler vs. the meter stick tools in Unit 1.