The Fourth Dimension, III

Last week we counted vertices, edges, faces, and cells on a hypercube.  Mainly, we thought by analogy — but how could we be sure our intuition was correct?  Today, we look at another say to make these same counts, and obtain the same results.  This suggests that maybe we were right all along….

So we begin by looking at the square and cube in a different way, and thinking (again) by analogy from there.

Let’s start with the number of edges on a square this time. We’ll start with one dimension less than the object we’re examining — later we’ll use the squares on a cube, and the cubes on a hypercube.  Now to count vertices, we note that each edge has two vertices — for 4 x 2 = 8 vertices.  But when we join the edges at vertices, two vertices merge, so we’ve overcounted by a factor of 2.  Thus, there are 8 / 2 = 4 vertices on a square (as we know).

What happens when we look at the cube in the same way?  Let’s start with the six squares.  To count edges, we see that each square on the cube has 4 edges, for a total of 6 x 4 = 24 edges.  But as with the vertices on the square, when we join two squares at their edges, two edges merge, so we’ve overcounted by a factor of 2.  This means there are 24 / 2 = 12 edges on the cube.

For vertices, we note that three squares meet at each vertex.  So there are  6 x 4 = 24 vertices on the 6 squares — but when we join the squares together, three vertices merge to one.  This means we’ve overcounted by a factor of 3, so there are 24 / 3 = 8 vertices on a cube.

Now begin with the eight cubes on a hypercube.  We note that the “8” comes from the sequence 2, 4, 6, 8,… for 2 vertices on a segment, 4 edges on a square, 6 squares on a cube, etc.  On the square, we saw that 2 vertices merged.  On the cube, we observed that 2 edges merged, then 3 vertices.  By analogy, on the hypercube, we should have 2 squares merging, 3 edges, and 4 vertices.  This pattern continues into higher dimensions as well.

Let’s use this analogy to check that we’ve counted correctly.  First, we count squares.  With 8 cubes, we have 8 x 6 = 48 squares.  But since the cubes meet square-to-square, we’ve overcounted by a factor of 2, so that there are just 48 / 2 = 24 squares on a hypercube.

Now on to the edges.  Since 8 cubes contribute 12 edges each, there are 96 edges in total.  But three cubes meet at each edge, so we’ve overcounted by a factor of 3.  This implies that there are in fact 96 / 3 = 32 edges on a hypercube.

Finally, we count the vertices.  With 8 vertices on each of 8 cubes, we have 64 vertices all together.  But four cubes meet at each vertex, so we have in fact overcounted by a factor of 4.  This results in just 64 / 4 = 16 vertices on a hypercube.

So we counted correctly!  Well, almost….  You might have noticed that we haven’t exactly defined what a hypercube is in a rigorous mathematical way.  Until we do, the best we can say is that we’ve counted something, although we don’t know precisely what they something is at the moment.  That will be a subject for a later post.

As you might expect, there are other ways to represent a hypercube besides the figure shown in last week’s post.  We chose that particular representation because of the way we were thinking by analogy.

We’ll look at two additional ways.  The first is analogous to looking at a cube face on, as shown below.

Day172Hypercube1

Of course the inner and outer squares are the same size on a cube — but we can’t help distorting faces of the cube when we make a two-dimensional sketch.  Four of the squares are distorted into trapezoids here.

Here is the analogous representation of the hypercube.

Day172Hypercube2

In this figure, the inner cube (in green with black edges) is in fact the same size as the outer cube (with blue edges); these cubes are directly opposite each other on the hypercube.  This is directly analogous to the inner and outer squares we saw in the earlier figure.

Further, note how the other six cubes are distorted into frustums of square pyramids — you can easily see the trapezoidal faces, which we also saw in the earlier figure.  If you look carefully, you can count the 16 vertices and 32 edges.  The 24 squares are a bit trickier — but begin with the 12 squares on the inner and outer cubes.  Each of the other 12 squares contains exactly one black edge and exactly one blue edge — they are going “out” from the black edges to the blue edges.  The perspective is different, but all the squares are there.

And of course there are the eight cubes — the inner cube, the outer cube, and the six frustums of square pyramids.  All the elements of the hypercube are indeed present, but again, in a different perspective.

I did save the best for last — my favorite representation of a hypercube, shown below.

Day172Hypercube3.png

I just love the symmetry of this image — the octagram inside the octagon.  If you look at the left image, you’ll see one of the eight cubes highlighted in blue.  When each of the eight cubes is transparently colored in the right image, you’ll see an interesting overlap of colors.

Now let’s count the vertices, edges, faces, and cells in this figure.  The 16 vertices are readily apparent in the inner octagram and the outer octagon.  The 32 edges can be seen by counting eight edges from both the octagram and octagon, and two additional edges connecting each vertex of the octagon to two vertices of the octagram.

The squares are a bit trickier here as well — but eight are easily visible as undistorted squares sharing one edge of the octagon.  But if you look carefully, you’ll also see 16 additional squares as rhombi in the figure.  Eight of these rhombi each share two edges with the octagon, and the eight others each share two edges with the octagram.  It might take staring for a minute, but they are all there.

And finally, there are the eight cubes. As seen in the left image, three consecutive edges of the octagon are enough to determine one of the cubes. Since we can take three consecutive edges of an octagon in exactly eight ways, we have found the eight cells on the hypercube.

So again, all the elements of a hypercube are present — it just takes looking from the right perspective to see them all.

Still remaining is to obtain the same counts by considering a rigorous definition of a hypercube.  That for a later post….

The Fourth Dimension, II

Last week, I introduced the idea of a fourth spatial dimension.  The typical question students ask is simply, “Where is it?”  These doesn’t seem to anywhere it can go — and in fact, since we live (insofar as we know it) in a three-dimensional world, there really isn’t anywhere it can go.

So the fourth spatial dimension must be, for us, an abstraction.  To think about it, we must somehow relate it to things we already know — in particular, geometry in dimensions zero, one, two, and three.  Therefore, to think about the tesseract — the four-dimensional analogue of the cube — we must somehow think by analogy.

In particular, we can think about how we go from a point (zero dimensions) to a line segment (one dimension), and then from a line segment to a square, a square to a cube, and then make the analogous leap from a three-dimensional cube to a four-dimensional tesseract, or hypercube.  We’ll look one way to do this in today’s post, and next week, we’ll see a different way.  It’s a good idea to look at this “new” fourth dimensional from different perspectives to make sure our thinking by analogy is accurate.

To create a line segment from a point, we think of it moving one unit (we need to be specific) along a “new” first dimension to create another point — these two points are vertices of the line segment, and one edge is created.

Now move this segment one unit along a second dimension which is perpendicular to the line segment, as shown below.

Day171a.png

Now let’s count.  We have two vertices and one edge for each of the two segments (the one at the bottom in black, and the one at the top in blue), and each vertex creates another line segment (shown in red above) as it moves from the bottom segment to the top segment.  This gives a total of four vertices and four edges on a square — as expected.

Thinking by analogy, we now imagine a square moving up along a perpendicular dimension, as shown below.

Day171b

Counting vertices, edges, and faces, we have 8 vertices, 8 edges, and 2 faces from the bottom and top squares, shown in black and blue, respectively.  Now each vertex of the bottom square creates an edge as it moves up (creating 4 more edges, shown in red), and each edge of the bottom square creates a new face (creating 4 new faces, shown in pale yellow).  This gives a total of 8 vertices, 12 edges, and 6 faces.  Notice the strategy: count the bottom and top figures, and then notice what is created by vertices and edges as they move along a perpendicular dimension.

Now it’s time to extend this strategy into the fourth dimension.  To do so, we need to imagine a cube moving out along a fourth spatial dimension — and of course, it is difficult to imagine because we are so used to our three-dimensional world.

Day171c.png

Look at the above figure.  Let’s think of the cube outlined in black as the base cube.  Move this cube out along a fourth spatial dimension — so that each vertex creates an edge (shown in red) as it moves to the top cube (shown in blue).

Before we start counting, we need to introduce a little terminology.  The four-dimensional analogue of a polyhedron is called a polytope, and in addition to vertices, edges, and faces, we have three-dimensional cells on a polytope.

Now let’s count the vertices, edges, faces, and cells on a hypercube in just the same way as in the previous two examples. For vertices, we have 16 total — 8 from the base cube, and 8 from the top cube.  For the edges, we have 12 each for the base and top cubes, and each vertex of the base cube creates a new edge (shown in red).  This gives a total of 12 + 12 + 8 = 32 edges on the hypercube.

What about faces?  Well, we have 6 faces each for the base and top cubes, and each edge of the base cube creates another square face (as it did when we created the cube).  This gives us 6 + 6 + 12 = 24 faces.  Finally, to count the cells, we have the base and top cubes, and each square face of the base cube moves out to create another cube (such as the one shown in yellow, with the square from the base cube shown in darker yellow).  This gives 2 + 6 = 8 cells (cubes) on the hypercube.

How can we be sure this is correct?  Next week, we’ll look at a different (though certainly related) way to count the number of vertices, edges, face, and cells on a hypercube.  We will see that we do in fact obtain the same counts.

As a final remark, we briefly look at Euler’s Formula in four dimensions.  Recall that V-E+F=2, where V, E, and F represent the numbers of vertices, edges, and faces on a convex polyhedron.  Now if C represents the number of cells on a polytope, the four-dimensional analogue of Euler’s Formula is

V-E+F-C=0.

In our case, we have the true statement

16-32+24-8=0.

We won’t go into more depth here, as that would take us quite a bit further afield.  But if you’re interested to know more, you can always look up Euler’s Formula in higher dimensions, and extensions which have to do with the topology of geometrical objects, which is fairly straightforward when the objects are convex.  When they’re nonconvex, the situation is decidedly more difficult.

Stay tuned for next week’s post, where we look at yet another way to count the numbers of vertices, edges, faces, and cells on our friendly tesseract!

The Fourth Dimension, I

I have found that the topics of infinity and the fourth dimension really do always pique students’ interest.  When I had a few moments left at the end of a class period, I could sometimes casually remark about one of these topics, and I immediately had the attention of the entire classroom.

One of my projects has been writing an introductory book on polyhedra, and I’m in the middle of a proposal to have the book published.  A draft chapter was on the fourth spatial dimension, so I thought I’d share it here.  As much as I love the topic, I rather surprised myself by looking at my blog index and finding I’d never talked about it before….

What follows is the draft chapter, slightly edited as a stand-alone series.  Enjoy!

“What is the fourth dimension?

No, it’s not time.

Well, maybe it is if you’re studying physics.  Even then, we have certain intuitive ideas about how time works.

For example, if I imagine that it’s 9:17 a.m. in San Francisco, then I know what time it is in any other city in the world.  In New York City, it must be 12:17 p.m., since I add three hours to convert to Eastern Standard Time.

This assumption is just fine for getting along in daily life — and as far as most people are concerned, this way of thinking about time is right.  But in fact, it works only because in our daily lives, we move around fairly slowly — at least compared to the speed of light.

To understand what happens when particles do move close to the speed of light, you need to study special relativity — and here, our ordinary intuitions about time are no longer valid.

But we’re interested in a fourth spatial dimension.  How is this possible?  Where is it?  We are so used to living in a three-dimensional world, the idea of a fourth spatial dimension seems rather fantastic.

In 1884, Edwin Abbott’s delightful novella Flatland was published (Abbott, Edwin A. Flatland: A Romance in Many Dimensions. New York: Dover Thrift Edition. 1992).  The protagonist was none other than A Square, an inquisitive four-sided being living in a purely two-dimensional world called Flatland.

He was chosen as the Flatlander to receive a visit from a Sphere on the eve of the Third Millenium.  This was an unnerving visit for A Square, since the Sphere kept suggesting he consider the direction upward, but there was no upward for A Square.  There was North, South, East, and West, but A Square just couldn’t fathom this direction, “upward.”

Finally, the Sphere lifted A Square out of his two-dimensional world to show him the glory of Space.  Quite a revelation!

We are in the same predicament as A Square when it comes to contemplating a fourth spatial dimension.  Yes, we can look forward, backward, to our left and right, up and down, but nowhere else.  It doesn’t seem that there is enough room for a fourth dimension.  Where would it be?

In a typical high school geometry class, you would likely have been introduced to points, lines, and planes — and perhaps were told that points are 0-dimensional, lines are 1-dimensional, planes are 2-dimensional, and that all objects of these types live in a 3-dimensional space.  But while there were infinitely many points, lines, and planes, there was only one 3-dimensional space.

And while we do not encounter a fourth spatial dimension on a daily basis, we don’t actually encounter points, lines, or planes, either.  Can we actually see a point if it has no length?  How could we possibly see a line if it has no width?  It would be invisible.  These geometrical ideas are in fact mathematical abstractions — and once we enter the world of mathematical abstraction, our universe becomes almost unimaginably vast. A. R. Forsyth wrote almost one hundred years ago (A. R. Forsyth, Geometry of Four Dimensions, Cambridge University Press, New York, 1930, p. vii.):

Mathematically, there is no impassable bar against adventure into spaces of more than three dimensions of experience.

To get a handle on how to imagine the fourth dimension, we’ll look at one of the most popular and well-know denizens of that rarefied world — the hypercube, also known as a tesseract.”

What follows is an extended example of “thinking by analogy,” which would make this post much longer than I usually allow myself.  So that’s where we’ll start next week!

Jagodina, Serbia: 2018

This week, I write from Jagodina, Serbia! I am here with my friend Dr. Snezana Lawrence (author of www.mathisgoodforyou.com and Mathematicians and Their Gods) presenting at a History of Mathematics in Mathematics Education conference at the University of Kragujevac.  Snezana secured funding for and helped organize this conference, which is intended to provide teachers and teacher educators resources for making mathematics more accessible to students by providing contexts for various mathematical ideas.

As mathematicians are well aware, mathematics is both a historical and cultural phenomenon.  I know relatively little about the history of mathematics, but I did do some research into the relationship between Kepler and Tycho in preparation for a talk on Kepler’s Laws and the Music of the Spheres.  The facts that Tycho was not only interested in astronomy, but was also a member of the Danish nobility (and hence had the money to design and create instruments which allowed for more accurate measurements than previously possible) were crucial in providing the data necessary for Kepler to deduce the laws which we know by his name today.  The story is of course much more involved; the point is that providing some context for statements such as Kepler’s Laws makes them more accessible to students rather than just stating them as if they were abstractions devoid of cultural and historical context.

My contribution was running a workshop on polyhedra (the Platonic solids in particular).  Participants were familiar with the Platonic solids at various levels, so I made sure there were nets available at varying levels of difficulty.  For example, we were going to discuss the geometry of fullerene molecules, so I also had some truncated icosahedron nets printed out as well.

The weather was particularly warm this weekend; plans were altered so we could be outside for much of the workshop.  So I moved everything I wanted to discuss using the computer to the first hour.  For example, to put our work into some context, I discussed my correspondence with Magnus Wenninger (which I excerpted in a series of blog posts earlier this year).

I also wanted to make a reference to Kepler’s Harmonices Mundi, as Snezana had mentioned Kepler earlier in the conference.  Among many other things, Kepler included a discussion of the Platonic solids.

Day169Kepler.png

Further, I wanted to discuss the fairly recent (certainly compared to Kepler) discovery of fullerenes, as this discovery led to an interesting, if relatively simple, result:  in a fullerene molecule, where each carbon is adjacent to exactly three others, and the carbon atoms come only in rings of five and six, then there must be exactly twelve rings of five carbons, regardless of how many rings of six there are.

This is actually not very difficult to prove using Euler’s formula (and I have done so in a previous post), but it is somewhat surprising the first time you hear it.  More difficult is to prove that there can be any number of hexagons except one; I didn’t go into this in any more detail, however.  But I did want to go to the Wikipedia article and put the question in some historical context.

After the lunch break, we proceeded to an outdoor classroom which we populated with two movable whiteboards.  First, I had participants build at least one of the Platonic solids using the double-tab method I learned from Magnus Wenninger.  We then moved on to an algebraic enumeration of the Platonic solids as an application of Euler’s formula (which was also mentioned earlier).  As some of the participants were involved with teaching younger students, I wanted to avoid spherical trigonometry.

Then, as mentioned above, we looked at fullerenes.  Since the technique involved here was similar to that used to enumerate the Platonic solids, after we defined the problem, I gave the participants time to solve it on their own.  Relatively quickly, one of the educators did prove that there were exactly twelve rings of pentagons, and presented her solution to the others.  This was a great way to end the afternoon; afterwards, we took a walking tour of the busier section of Jagodina.

Of course one interesting aspect of attending any international conference is learning about the similarities and differences between education in different countries.  In Serbia as well as the United States, teachers are not paid as well as other professions.  But mathematicians have fewer opportunities for employment than they do in the US, so they are more likely to go into teaching as a career.

One striking difference revolves around school subjects and the corresponding teacher training.  As early as fifth grade, the sciences are separated as subjects.  All students take biology and informatics (computer science) beginning in the fifth grade, physics beginning in the sixth grade, and chemistry beginning in the seventh grade.  Moreover, teachers of mathematics, biology, physics, and chemistry have to major in that subject to teach in grade five and beyond.  (Up to the fourth grade, there is just one teacher for all subjects except foreign languages.)

A suggestion was once made to change this in mathematics: just allow mathematics teachers in middle school grades to take a few courses, similar to what we could call a mathematics endorsement here.  This was strongly rejected by those responsible for training mathematics teachers.  We can learn a lesson from this, as many teachers of middle school mathematics in the US are not adequately prepared for their classrooms.  However, as the teaching of informatics is a fairly recent addition to the curriculum, a major in computer science is not necessary to teach informatics in the middle school grades.

Another significant difference is how mathematics is taught at university.  Each major in the sciences has their own sequence of mathematics courses, taught by faculty in that department; physics typically has a three-semester sequence, and chemistry a two-semester sequence.  So there is no “one size fits all” calculus sequence, as is commonplace in American universities.

One consequence of this is that there is no gateway proof-writing course in the mathematics major.  Theory and proof are integrated into mathematics courses from the very beginning.  Thus, to earn the highest marks in an introductory calculus course, a student must show an ability to think abstractly and write mathematical arguments.  This would seem like such a luxury for the typical American mathematics professor – a Calculus I course consisting entirely of math majors!  But this is just standard operating procedure in universities in Serbia.

So my first experience in Serbia was an enjoyable one, and I learned a lot about the culture and education.  I look forward to a few days in Belgrade before returning to the US later on in the week.  I leave you with this interesting piece of found art I saw walking down the main street of Jagodina — you truly can never tell what surprises await when visiting somewhere new!

IMAG9927.jpg

On Assessment, V

Last week, I ended with a sample exam I might give a calculus course which included both Skills problems and Conceptual problems.  Before presenting the final installment of this series on assessment, I thought I’d take a few moments to discuss the genesis of this exam format.

Again, the assumption here is that we are working in a more traditional system, where students must be assigned grades, and these grades must in large part be based on performance on exams.

Given IMSA’s statements about the advanced nature of their curriculum, I had concerns about the fairly traditional exams we gave in mathematics.  In my mind, there was little to distinguish our exams from those given in any other rigorous calculus course.

The reason given to me by other mathematics faculty was that there just wasn’t time in a roughly hour-long exam to assess conceptual understanding.  I wasn’t convinced, and I started thinking of an alternative.  I did agree, however, that it wouldn’t work to have a conceptual question on an exam which might take half the exam period for a significant fraction of the students to complete.

What I finally settled upon was including a range of conceptual problems for which students only needed to provide a reasonable approach to solving.  If you chose a conceptual problem which happened to be centered on a student’s weakness, you wouldn’t be able to assess a broader conceptual understanding.  And if you insisted the problem be worked completely through, you encountered significant time constraints.

I’d like to share one last anecdote.  I recall a parent visitation day one Saturday, which happened to be the day after I gave a calculus exam.  Two of the parents approached me after the session and told me how much their son or daughter enjoyed my exam.  This indicated to me that for the student who can perform the routine procedures easily, they want to be challenged to think outside the box, and indeed they thrive on such challenge.  Shouldn’t we, as educators, find ways to stimulate all of our students, rather than be content with having students in the middle earn their B’s, making sure the struggling students earn their C’s, and relegating the very capable students to a sustained boredom?

And now for the last installment….

“Where does this bring us? Here are some key points as I see them.

  1.  We should move away from assigning grades punitively.
  2. We should reconsider the “point'”system of evaluating student performance.  Referring to the TIMSS (Third International Mathematics and Science Study):  “In our study, teachers were asked what ‘main thing’ they wanted students to learn from the lesson.  Sixty-one percent of U.S. teachers described skills they wanted their students to learn.  They wanted students to be able to perform a procedure, solve a particular kind of problem, and so on….On the same questionnaire, 73 percent of Japanese teachers said that the main thing they wanted their students to learn from the lesson was to think about things in a new way, such as to see new relationships between mathematical ideas.” (Stigler and Hiebert, The Teaching Gap, ISBN 0-684-85274-8, pp. 89-90.)  A point system reflects the assessment of procedural knowledge.
  3. “We can think of all assessment uses as falling into one of two general categories — assessments FOR learning and assessments OF learning.”  (From an internal document distributed to mathematics teachers at IMSA.)  But why?  The distinction is artificial.  There are many other ways to compartmentalize assessments, such as timed/untimed, individual/group, skill/conceptual, procedural/relational, short-term/long-term, etc.  The main argument for focusing on the “for/or” distinction is its relationship to student motivation — but we are given no context for it.  I suggest that our typical IMSA student is highly motivated — certainly in relation to the average student in a typical high school classroom.
  4. We should consider the assignment of letter grades in general.  Right now, it would be impractical to suggest that we have formal written evaluations of each student in each class.  But is it desirable?  And if so, what resources are necessary to support such a system?
  5. We should discuss the assessment of problem-solving.

Will any of these suggestions help to illuminate the power of ideas?  I’m not sure.  With the current need to assign grades, and their current cultural meaning and importance — especially when it comes to applying to college — there will be the necessary compromises in the classroom.  I realize that many suggestions are of the “move away” rather than the “move toward” type.  But I suppose that if there is something I am moving toward, it’s giving students at all levels more of a BC Fast-Track experience regardless of the depth of content.

This means actively moving toward a classroom environment where earning good grades is subordinate to learning complex concepts.  Of course the two are not mutually exclusive — but I’d rather have students earn good grades because they learned, rather than learn in order to get good grades.

Of course many issues brought up in these remarks have been left hanging or only tentatively developed.  These brief comments are meant to suggest questions for discussion, not definitive answers.

I can’t resist ending with the following challenge from Maslow: “In order to be able to choose in accord with his own nature and develop it, the child must be permitted to retain the subjective experiences of delight and boredom, as the criteria of the correct choice for him. The alternative criterion is making the choice in terms of the wish of another person. The Self is lost when this happens.  (Maslow, source cited earlier, p. 58.)  Is it possible to create a mathematics curriculum which can survive this test of course selection?”

Thanks for staying with this series!  No, there is no simple resolution to any of the issues described in this essay.  But that doesn’t mean we shouldn’t be involved in a conversation about them….

On Assessment, IV

Last week, I had ended with an interpretation for an A–D grading scale, shown below (here is a link to last week’s post for reference).

Day166grades

I remind you that this scale is not ideal; the purpose was to come up with some system of assigning grades which wasn’t punitive, but rather which motivated students to learn concepts rather than to avoid losing points on exams.

We continue with a discussion of how to use such a system in practice.

“Now let’s consider this in the context of an exam.  The first part of an exam is a skills portion, with, say, ten short problems of roughly equal length.  Expectations for this part of the exam [meaning a grade of B] are seven problems “essentially” correct, and four problems completely correct.  These expectations are written on the exam for students to see.

Are these expectations too low?  Perhaps.  But then an A [refer to the chart below] means eight problems “essentially” correct, with at least five completely correct. Of course we must ask what it means for a problem to be “essentially” correct — but when in doubt, err on behalf of the student.  (Students rarely suggest that their scores be lowered.)

Then grading is actually somewhat easier, and grades can be assigned as follows, with the abbreviations EC and CC meaning essentially correct and completely correct, respectively (for simplicity, a grade of C is assigned for all other cases not accounted for):

Day167grades

Now this eliminates the need for partial credit — but does require a judgment as to what “essentially correct” means.

This also makes grading much easier.  I would suggest that each problem be marked as “EC,” “CC,” or left blank.  Few comments, if any are necessary.  This is the approach I have taken in BC Fast-Track, and it encourages further learning as it leaves the student in the position of needing to work through their mistakes.

I would have students keep a section of their notebooks for exams and revisions, and there they can keep their reworked problems, should they choose to do so.  Then — as I did in BC Fast-Track — students could visit me periodically with their notebooks and I can take a look at their ongoing progress.  This “additional” work, if sufficiently well done, could boost their grade at the end of the semester.

I think this could have the same effect it did in BC Fast-Track — exams were easier and more enjoyable to grade.  But there were more discussions in my office about reworked exams and sources of error that were initiated by the students themselves, and these discussions were not about points, but about concepts.

Now what about the part of the exam which is intended to be more conceptual?  Let us suppose that there are three problems, roughly comparable in length, and of various difficulties.  Then grades might be assigned as follows:

Day167grades2

More details about how this would fit in a classroom environment may be found in a later document [I cannot recall which document is being referred to here].  But this system allows for a more qualitative approach to grading.  Performance expectations are also clearer, but such expectations depend critically upon the nature of the problems given.  Moreover, grades are not assigned punitively, but the emphasis is on doing problems completely and correctly.

For an example, below could be a set of ten skills problems and three conceptual questions for a basic assessment on the rules of differentiation.  This would a 70-minute assessment.  Given expectations for completely correct problems, I think this is reasonable.

Skills questions:

1.  Evaluate

\lim_{x\to\pi/4}\dfrac{\tan x-1}{x-\pi/4}.

2.  If f(x)=e^x\cos x, find f^\prime(x).

3.  Find

\dfrac d{dx}\dfrac 7{\sqrt{x^3}}.

4.  Using the quotient rule, find

\dfrac d{dx}\dfrac{x^3}{\sin x},

simplifying as much as possible.

5.  Find the derivative of f(x)=\left(\sin\sqrt x\right)^{\!2}.

6.  Find the equation of the tangent line to h(x)=\sec(2x) at x=\pi/6.

7.  Using a definition of the derivative, find the derivative of p(x)=x^2-x.

8.  Assume that f and g are differentiable functions. Find

\dfrac d{dx}f(g(x^2)).

9.  Find the derivative of q(x)=x^2e^x\cot x.

10.  Let f be the greatest integer function. Using the definition of the derivative, determine whether or not the derivative exists at x=0.

Conceptual questions:

1.  Using the product rule, find

\dfrac d{dx}f(x)(g(x))^{-1}.

Explain your result.

2.  Suppose that the line y=6x+a is tangent to both f(x)=x^2+b and g(x)=x^3+3x. Find a and b.

3.  Suppose that f is a differentiable function. Discuss the following limit:

\lim_{h\to0}\dfrac{f(x+2h)-f(x-h)}{3h}.

Stay tuned for next week’s final installment of this series on assessment….

On Assessment, III

Today, I’ll continue with the discussion of assessment I began a few weeks ago.  Last week, I ended with the observation that “The performance of our students determines our expectations, rather than the other way around.”  Before continuing, I just want to make clear the context for that remark.  When I wrote this essay, IMSA was fully funded by the state of Illinois.

What would it look like to legislators — who approved our funding — if you brought in the top students (relative to their local regions), and then they regularly received low grades, or worse yet, flunked out?  Not good.  And because the admissions process was fairly involved, would this indicate a major flaw in this process?

Yes, some students couldn’t handle the high-pressure academic environment.  But I know that in several of my classes, a student needed to work hard not to earn at least a C.  In other words, showing up, handing in homework on time, and doing a reasonable amount of studying for quizzes and exams would usually guarantee a grade of at least a C.

Now is not the time to dive more deeply into this artificial adjustment of expectations, but I did want to mention that this issue is a significant one, even at presumably elite schools for mathematics and science students.

So to continue with the essay….

“Now there is a natural give-and-take between evaluating student performance and setting expectations.  And, of course, the above remarks are nothing but generalizations.  But they illustrate some of the important issues at hand, and may bear fruitful discussion.

Moving to more concrete issues, I believe that the assignment of letter grades on exams in BC Fast-Track [recall, this was the colloquial name for the Honors Calculus sequence] was, on the whole, successful.  Without going into unnecessary detail, the classroom environment was such that the assigned grades were meaningful to the students.  To give a few examples, I assigned a grade of A+ for truly outstanding work, perhaps only a half-dozen times throughout the entire semester.  The students knew this, and so that accolade truly meant something.

Moreover, an A meant something as well.  It was truly rewarding to see the real pride of a student who, used to earning grades in the B range, began to earn the occasional, or perhaps more frequent, A.  Admittedly, students who made it to the second semester were essentially guaranteed a grade of no lower than a B-.  But this seemed to make an A that much more meaningful.

So student exams had two letter grades on them — one for the skills portion of the exam, and one for the conceptual portion of the exam.  No points were assigned, and few comments were made.  Students were expected to rework problems on which they made errors.

I bring up this point because I think this system of assigning grades really did motivate students to learn calculus rather than accumulate points.  This is the critical issue: I suggest that the way we assign grades does little to disabuse many students that taking a mathematics course is about accumulating sufficiently many — or losing sufficiently few — points.

Let’s take a particular example.  The past few semesters, I stopped assigning half-points on assessments [the usual practice at IMSA].  I might forgive a sign error now and then, but too many on a single assessment would warrant a point or two off.

In the past, I simply considered a sign error as a half-point off.  And so it was.  But consider that without being able to occasionally perform fairly involved calculations, it is not possible to become a successful mathematician.  Attention to detail is as important in mathematics as it is in any number of other disciplines, and we try to develop that skill punitively — you don’t attend to detail, and we will take off points.

Of course one might argue that points are given for work well done — but any of us could, I think, agree that when discussing the grading of an exam, it’s how many points off for a particular type of error that is discussed as much as, or even more than, how many points are given for work correctly done.

And so the idea of “partial credit” is born.  Perhaps now is not the place to begin this discussion, but consider that a student might meet expectations (that is, earn a B) without ever having done an entire complex problem on an assessment completely correctly.  (Some teachers have even gone so far as to give no partial credit.  See On Partial Credit, Letter to the Editor, MAA Focus, February 2002, p. 17.)

Why this system of points and partial credit?  One may speculate as to its origins, and there is controversy even now about its use on standardized exams.  But I cannot help feeling that one function of partial credit is that it allows a teacher to defend the assignment of a particular grade.  “Every sign error is a half-point off.  That’s why you got a B+ instead of an A-.   I have to use the exact same scale for everyone in order to be fair.

But doesn’t this simply shift the responsibility for the grade onto a rubric?  I suggest that many of us would feel competent to take a set of calculus exams — with names removed — and within five or ten minutes, separate out all the A papers.  Of course this is subjective — but no less subjective than saying that this problem is worth six points while another is worth ten, or that sign errors are a half-point off, unless, of course, the derivative of the cosine is taken incorrectly, in which case it’s a whole point.

Thus the assigning of points is no more “objective” than giving a letter grade.  As I’m sure that anyone who has graded a complex word problem based on an assignment of points can attest to.  Consider the student who has the entire procedure correct, but because of a few algebra errors, has no intermediate calculation correct.  The problem is worth ten points.  How many points should the student receive?

Well, of course, you say you’d have to see the problem first.  But I say, no.  The student receives a C.  Having no intermediate calculations correct demonstrates — regardless of what else — that the student has not met expectations.

So is it possible to avoid points altogether?  Perhaps.  Consider the following grading system:

Day166grades.png

The essay continues in the next installment of On Assessment with a discussion of how to implement this system in practice.