Check out this recent research:
Johnson, H. L. (2015) Secondary students’ quantification of ratio and rate: A framework for reasoning about change in covarying quantities. Mathematical Thinking and Learning, 17(1), 64-90.
http://www.tandfonline.com/doi/full/10.1080/10986065.2015.981946
Johnson, H. L. (2015). Together yet separate: Students’ associating amounts of change in quantities involved in rate of change. Educational Studies in Mathematics, 1-22. doi: 10.1007/s10649-014-9590-y
http://link.springer.com/article/10.1007/s10649-014-9590-y?sa_campaign=email/event/articleAuthor/onlineFirst