Disaster By Design

Much research of human beings tends to focus on aggregates and averages. But humans come in various shapes and sizes; they have various skills and weaknesses; they do not all travel the same road at the same pace at the same age. One would not issue the same size of clothing to every first-grader; it could come up short on some, and sag on others. Why, then, must they read from the same page of the same primers? Why do the same single-digit addition, from the same page of the same textbook?