This thesis examined middle school students' current understanding of variability using a constructed response item assessment question. Variability is an essential concept in the teaching and learning of statistics. However, many students have difficulty with the concept of variability especially when constructing boxplots. Using a framework methodology with pre-defined levels from a middle school student thinking and reasoning matrix, student responses were examined. The results show that students have difficulty with concept of variability in four areas: graph knowledge specifically related to analyzing and interpreting boxplots, communicating inferences, contextual knowledge and misinterpretation of averages and distributions. These themes provide a foundation for discussion of how teachers and research can design curriculum and instruction to engage students in designed learning activities that will help enhance their understanding of variability. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]