Friday, October 17, 2008

An Excessive Fondness for Fractions
- by Bill Marsh, Ph.D.

An Excessive Fondness for Fractions

Bill Marsh, Ph.D.
October 16, 2008

“The importance of fractions to mathematics cannot be overstated. ... For middle and high school students, real numbers are mostly taken for granted ... The point is that fractions are an essential intermediary step between whole number and real numbers. ...It is impossible to overstate the importance of fractions. Numbers and geometry are at the heart of mathematics, and fractions are required for both. You can’t do mathematics without an understanding of fractions and their operations.” -- Mathematician W. Stephen Wilson in “Review of Mathematical Soundness”, 2008.

“[D]oes not the difficulty begin with fractions? Should we have such a notion of these numbers if we did not previously know a matter which we conceive of as infinitely divisible – i.e., as a continuum?” -- French mathematician and philosopher Henri Poincare in Science and Hypothesis, 1903.

“The device beyond praise that visualizes magnitudes and at the same time the natural numbers articulating them is the number line, where initially only the natural numbers are individualized and named.” -- German/Dutch mathematician and educator Hans Freudenthal in The Didactical Phenomenology of Mathematical Structures, 1983.

“For students of all ages, definitions of basic mathematical concepts have to be framed with care: not too formal, not too informal. The New Math movement gave rigorous definitions starting with whole numbers, i.e., 5 is the collection of all sets that contain 5 elements, that were out of touch with children’s and parents’ mathematical experience. The common sense notion of a whole number as a counting number, to count how many items are in a collection, provides an adequate informal definition in the early grades. Later, in anticipation of rational and real numbers, whole numbers can be identified with appropriate points on the number line.” -- Mathematician Alan Tucker in “Preparing for Fractions”, 2006 PCMI Workshop.

In his review for the Washington State Board of Education, Professor Wilson says "The importance of fractions to mathematics cannot be overstated." He then proceeds to do what he says is impossible.

He says "The point is that fractions are an essential intermediary step between whole number and real numbers." Some fractions, yes, but not all of them. All you need is a dense subset of the reals. Two such suggest themselves: decimals, whose denominators are powers of ten, and the fractions used by carpenters and computers, those with denominators a power of two.

Following the excellent advice on definitions given by Professor Tucker, first or second graders can define, think of, and use "measuring numbers" as "good names for dots on number lines." To see a six-year-old explain in less than six minutes a "naming trick" that gives a notation system for the non-negative real numbers, go to

Professor Wilson says: "Numbers and geometry are at the heart of mathematics, and fractions are required for both." Fractions are necessary in geometry, but we all know that they are not sufficient. Neither the circumference of the unit circle nor the diagonal of the unit square has a length expressible as a fraction. For geometry and measuring, fractions are too complicated and not good enough. There are both too many of them and too few.

Fifty years after reading Science and Hypothesis what I most remember is the model of non-Euclidean geometry inside Euclidean, which shows that we can deny an axiom. I think that the answers that Poincare expected to the questions above means that we can deny the widely held assumption that fractions are what should come next after the counting numbers as children learn arithmetic. In k-4 arithmetic, we can be natural, get real, not rationalize, and not be negative.


Anonymous said...

That's been known for some time, I have an extended algebra class now, with about a 1/3 who were failures in algebra. These kids will be seniors before they are eligible for informal geometry.

Today I saw progress reports and I was surprised at the number of students failing informal geometry.

The counselors seem not to know the difference between algebra and extended algebra. Nearly all of these kids have received little or no instruction with fractions. The fact they are in high school rules out one poor teacher.

The problem is systemic and has to be corrected. Otherwise, hs teachers will need to be prepared to teach fractions and there is not very much available that is appropriate for this age group.

We started some fraction instruction today using tangrams. If a triangle = 1/4 find the shape of the quadrilateral that is 2 1/4.

That was a struggle for my seniors.

dan dempsey said...

The fraction difficult was certainly called out by the national math panel. When kids come through many of the reform math k-8 programs most seem to do so with little understanding or competence in dealing with fractions.

Anonymous said...

Ah, what the heck! Fractions are not really important. After all, the person making this statement was still able to get her point across,

"In my opinion, two-thirds of the students are doing just fine in math class. The other 15% just needs a little more help."

dan dempsey said...

I sure do agree with the above comment....

Lemm me see here:

2/3 = 66 2/3%

66 2/3% + 15% =

81 2/3%

I Changed the fraction to a percent and added the other percent to get 81 2/3%.

81 2/3% is everybody in this case so there is no one left out.

The above answer sure should get full WASL credit.