The development of school students' understanding of comparing two data sets is explored through responses of students in individual interview settings. Eighty-eight students in grades 3 to 9 were presented with data sets in graphical form for comparison. Student responses were analysed according to a developmental cycle which was repeated in two contexts: one where the numbers of values in the data sets were the same and the other where they were different. Strategies observed within the developmental cycles were visual, numerical, or a combination of the two. The correctness of outcomes associated with using and combining these strategies varied depending upon the task and the developmental level of the response. Implications for teachers, educational planners and researchers are discussed in relation to the beginning of statistical inference during the school years.