As a principal form of statistical thinking, consideration of variation impacts on all aspects of statistics. There has been extensive research about students’ reasoning about variation but little research focusing on helping students model variation as a combination of explained and unexplained variation. A study analysed responses to a measurement instrument that was developed to assess tertiary students’ informal reasoning about variation, focusing on explained and unexplained variation. Selected students were also interviewed. This paper reports the analysis of the responses that informed the refinement of a framework that describes six components of reasoning about explained and unexplained variation. Implications for researchers and educators will also be discussed.