This chapter analyses and discusses nine groups of upper secondary students’ work on the question “How many red books are there in the library?”. The students devised and implemented a plan, collected data, calculated estimates and reflected on what aspects and factors might have influenced their results caused by their adopted strategy. The analysis of the students’ work focused on reconstructing and categorizing the models the students devised and implemented, as well as the sources and types of variability that the activity elicited. The results show how the central statistical idea of variability is manifested in the models developed and implemented by the students, and how these can be further explored and applied as central and bearing ideas for organizing a whole section of statistics at the upper secondary level.