In differentiated classrooms, teachers begin where students are, not the front of a curriculum guide.
The central job of schools is to maximize the capacity of each students.
We need to understand where are students are at any point during a unit - in other words, what each student actually knows, understands, and can do at a given time based on the content goals we've established.
Readiness is a student’s entry point relative to a particular understanding or skill.
We aren't quite sure what we're trying to differentiate, and therefore can't quite see how to do it other than giving some students more to cover and some less. That rarely works.