Readiness-based diferentiation - "Our best understanding of how people learn is that they begin with past knowledge, understanding, and skill and extend these to new levels of complexit or sophistication. Further, we learn best when the work we do is a little too hard for us. What that means is that we have a sense of both that the task calls for and the gaps in our capacity to do e=what it asks of us. When these gaps are not present (in other words, when we can do a task effortlessly), we do not learn because we do not streth what we already know. Similarly, when the gaps are too great, we cannot span the, and do not learn. Learning takes place when we have to stretch a managable amount and do so. Readiness-dased differentiation attempts to design student work at varied levels of challenge so that each student has to stretch a manageable amount and is supported in doing so."
C. Tomlinson & C. Strickland (2005) Differentiation in Practice: A Resource Guide for Differentiating Curriculum Grades 9-12. ASCD.