I am interested in studying students reasoning while learning a measure of variation and how making connections to the measures of center can improve retention. How is variation, more specifically the Mean Absolute Deviation (MAD) taught in sixth grade, and has there been any research that might be related?

I don't yet have a clear hypothesis, but a muddy one might be  that a group provided with extended relational instruction (where connections are made between a measure of center and the MAD by both computing the MAD as a percentage of the measure of center as well as computing an expected range of values) will perform better on posttest and retention evaluation assessments for center, variation and proportional reasoning than another group (with the same teacher) who are provided with computational reinforcement for their additional instruction time. 

It seems most instructional material I have seen (NC Mathematics "How MAD are you and LearnZillion.com for example) has the student compute the MAD, but not connect or relate it to to the center by computing an expected value range or expressing it as a percentage.

Thanks in advance for your assistance!!!