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12/30/2013 — Happy Derangement Day!

Today we celebrate a Derangement Day!  Usually I call days like today a permutation day because the digits of the day and month can be rearranged to form the year, but there’s something extra special about today’s date:

derangement day

The numbers of the month and day are a derangement of the year:  that is, they are a permutation of the digits of the year in which no digit remains in its original place!

Derangements pop up in some interesting places, and are connected to many rich mathematical ideas.   The question “How many derangements of n objects are there?” is a fun and classic application of the principle of inclusion-exclusion.  Derangements also figure in to some calculations of e and rook polynomials.

So enjoy Derangement Day!  Today, it’s ok to be totally out of order.

2012: Happy New Number!

Welcome 2012!  It will be hard to measure up to the numerous numerical nuances of 2011, but the number 2012 does possess some interesting properties.

The prime factorization of 2012 is noteworthy:

Not only does 2012 have only two distinct prime factors, but the prime factor 503 is rather large.  In fact, 2012 is an unusual number, in that its largest prime factor is greater than its square root.

Also, since 2012 has exactly three prime factors, it is considered triprime (or 3-semiprime).

We might not enjoy as many special numerical days this year (like palindrome days or permutation days), but I do look forward to writing 2012 for the next 365 days!

11022011

I noticed that two recent dates had the property that the day-month combination was just a permutation of the year.

This struck me as a rare occurrence.  And then I realized that we had an honest-to-goodness palindrome date coming up!

This is definitely a rare occurrence!  In celebration, I recommend that you do something forwards and then backwards.

I suppose the question now is, “When is the next palindrome day?”

Are These Tests Any Good? Part 2

This is the second entry in a series that examines the test quality of the New York State Math Regents Exams.  In the on-going debate about using student test scores to evaluate teachers (and schools, and the students themselves), the issue of test quality rarely comes up.  And the issue is crucial:  if the tests are ill-conceived, poorly constructed, and erroneous, how legitimate can they be as measures of teaching and learning?

In Part 1 of this series I looked at three questions that demonstrated a significant lack of mathematical understanding on the part of the exam writers.  Here, in Part 2, I will look at three examples of poorly designed questions.

The first is from the 2011 Integrated Algebra Regents:  how many different ways can five books be arranged on a shelf?

This simple question looks innocent enough, and I imagine most students would get it “right”.  Unfortunately, they’ll get it “right,” not by answering the question that’s been posed, but by answering the question the exam writers meant to ask.

How many different ways are there to arrange five books on a shelf?  A lot.  You can stack them vertically, horizontally, diagonally.  You can put them in different orders; you can have the spines facing out, or in.  You could stand them up like little tents.  You could arrange each book in a different way.  The correct answer to this question is probably “as many ways as you could possibly imagine”.  In fact, exploring this question in an open-ended, creative way might actually be fun, and mathematically compelling to boot.

But students are trained to turn off their creativity and give the answer that the tester wants to hear.  A skilled test-taker sees “How many ways can five books be arranged on a shelf?” and translates it into  “If I ignore everything I know about books and bookshelves, stand all the books upright in the normal way, don’t rotate, turn or otherwise deviate from how books in math problems are supposed to behave, then how many ways can I arrange them?”

This question is only partly assessing the student’s ability to identify and count permutations.  This question mostly tests whether the student understands what “normal” math problems are supposed to look like.

This problem is an ineffective assessment tool, but there’s something even worse about it.  Problems like this, of which there are many, teach students a terrible lesson:  thinking creatively will get you into trouble.  This is not something we want to be teaching.

Here’s a question from the 2011 Algebra II and Trigonometry exam:

Solving equations is one of the most important skills in math, and this question pertains to a particular method (completing the square) used to solve a particular kind of equation (quadratic).  But instead of simply asking the student to solve the problem using this method, the question asks something like “if this procedure is executed normally, what number will be written down in step four?”.

This is not testing the student’s ability to do math; instead, it’s testing whether or not they understand what “normal” math looks like.  There are many ways to solve equations, and there are many ways a student might use this method.  Whether it looks exactly like what the teacher did, or what the book did, isn’t especially relevant.  So why is that being tested?  And like the question above, this reinforces the idea that thinking creatively can be dangerous by insisting that students see the “normal” solution as the only correct one.

Finally, here’s a problem from the 2011 Geometry Regents:

Once again, the student is not being tested on their knowledge of a concept or on their ability to perform a task.  Instead, they’re being tested on whether or not they recognize what “normal” math looks like, and that’s just not something worth testing.  There are lots of legitimate ways to construct a perpendicular bisector:  why are we testing whether the student recognizes if the “normal” way has been used?

These three problems showcase some of the dangers inherent in standardized testing.  Questions like these, and the tests built from them, discourage creative thinking;  they send students the message that there is only one right way to do things; they reinforce the idea that the “correct” answer is whatever the tester, or teacher, wants to hear; and they de-emphasize real skills and understanding.

At their worst, these tests may not just be poor measures of real learning and teaching; they may actually be an obstacle to real learning and teaching.

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