The objective of this research was to better understand the transfer of learning between different variations of pre-algebra problems. While the authors could have addressed a specific variation that might address transfer, they were interested in developing a general model of transfer, so we gathered data from multiple problem types and their variants over the course of learning (see Setting, Intervention and Design sections). They gathered their data from the classroom but used randomization of item selection and sequence for each student because they were concerned about existing data containing various sources of bias (Shadish & Cook, 2009). Their method, which has been called "in vivo experimentation", blends attention to experimental method with attention to the real life issues of classroom learning, such as motivation issues, attendance and classroom distractions compared to the lab. (Contains 1 figure and 1 table.)