Unit 1, Inventing Displays, engages students in a simple process: Measure the length of an object, such as the arm-span of a person or the perimeter of a table. Each student independently measures the same attribute. The tool provided for measurement, a 15 cm ruler, ensures that students’ measurements will be different due to chance variation in how they measure. The aim is to help students understand that variability is the result of a repeated process. Here each person’s measurement of the length is a repetition of the process of measuring, which includes moving the ruler. If time allows, students measure the same attribute again with a meter stick or tape measure, and these additional measurements are set aside for future use. Measuring with a meter stick or tape measure provides students with an opportunity to relate a change in process with a change in the resulting variability of the measurements. Students create displays of the ruler data to illustrate "something they noticed" about the collection of data from the entire class. Students then compare the various displays that they make for aspects of the data that are revealed and concealed by each data display. The shapeof the data visible in the display results from designers’ decisions about how to structure the data. Important mathematical means for structuring data include ordering, counting, grouping, and using the measurement scale of the batch of measurements. Milestones of student progress are illustrated by the Data Display (DaD) and Meta-Representational Competency (MRC) constructs.