Murray says on page 21 in his book Real Education: "This point
needs emphasizing. Educational measures such as test scores and
grades tend to make differences among schoolchildren look as
through they are ones of degree when in reality some of them are
differences in kind. For example, a timed math test limited to
problems of addition and subtraction, administered to a random
cross-section of fourth-graders, yield scores that place
children along a continuum distrubuted in a shape resembling a
bell curve. Those scores appropriately reflect differences in
degree: Some fourth-graders can add and subtract faster and more
accurately than others, but they are all doing the same thing
and almost all children can be taught to add and subtract to
some degree. The same is not true of calculus. If all children
were put on a mathematics track that took them through calculus,
and then were given a test of calculus problems, the resulting
scores would not look like a bell curve. For a large proportion
of children, the sco