Monthly Archives: March 2015

Statistics Arts and Crafts

The Chi-Squared chapter in AP Statistics provides a welcome diversion from the means and proportions tests which dominate hypothesis test conversations. After a few tweets last week about a clay die activity I use, there were many requests for this post – and I don’t like to disappoint my stats friends! I first heard of this activity from Beth Benzing, who is part of our local PASTA (Philly Area Stats Teachers) group, and who shares her many professional development sessions on her school website. I’ve added a few wrinkles, but the concept is all Beth’s.

ACTIVITY SUMMARY: students make their own clay dice, then roll their dice to assess the “fairness” of the die. The chi-squared statistic is introduced and used to assess fairness.

clayYou’ll need to go out to your local arts and crafts store and buy a tub of air-dry clay. The day before this activity, my students took their two-sample hypothesis tests.  As they completed the test, I gave each a hunk of clay and instructions to make a die – reminding them that opposite sides of a die sum to 7. Completed dice are placed on index cards with the students names and left to dry. Overnight is sufficient drying time for nice, solid dice, and the die farm was shared in a tweet, which led to some stats jealousy:

The next day, students were handed this Clay Dice worksheet to record data in our die rolling experiment.

In part 1, students rolled their die 60 times (ideal for computing expected counts), recorded their rolls and computed the chi-squared statistic by hand / formula. This was our first experience with this new statistic, and it was easy to see how larger deviations from the expected cause this statistic to grow, and also the property that chi-squared must always be postivie (or, in rare instances, zero).

Students then contributed their chi-squared statistic to a class graph. I keep bingo daubers around my classroom to make these quick graphs. After all students shared their point, I asked students to think about how much evidence would cause one to think a die was NOT fair – just how big does that chi-squared number need to be? I was thrilled that students volunteered numbers like 11,12,13….they have generated a “feel” for significance. With 5 degrees of freedom, the critical value is 11.07, which I did not share on the graph here until afterwards.


In part 2, I wanted students to experience the same statistic through a truly “random” die. Using the RandInt feature on our calculators, students generated 60 random rolls, computed the chi-squared statistic, and shared their findings on a new dotplot.  The results were striking:

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In stats, variability is everywhere, and activities don’t often provide the results we hope will occur. This is one of those rare occasions where things fell nicely into place. None of the RandInt dice exceeded the critical value, and we had a number of clay dice which clearly need to go back to the die factory.

Introducing Discovery Hour with Codebreaking

Our school has been on a semester block schedule for over 20 years, with some tweaks made to accomodate building size, AP courses and electives. But this year brought a major schedule change, and an opportunity to think about how we use time to engage students. After periods 1 and 2 (each 75 minutes), all students move into something called HATS period. The acronym stands for Hatters Achieving Targeted Success, and during the period students have a lunch period, along with assigned time with teachers. It’s a great mid-day block for students to touch base with activities and clubs, seek help, make up work, and our RTII team has utilized the time to meet formally with students and facilitate individual help sessions.

I saw an opportunity to engage students in meaningful activities during this time, and have started Hatters Discovery Hour – modeled after the Genius Hour concept many elementary and middle schools offer. My thought is that so many of our teachers have awesome ideas to share which don’t quite fit class time. Also, it’s an opportunity for students to experience teachers they may not cross paths with during their high school career. Let’s build more connections!

The past 2 months have seen some fascinating offerings.  Our No Place for Hate Team has used Discovery Hour to facilitate open discussions on race relations.  Meanwhile, a science teacher shared his experiences working as an EMT in a medical diagnosis session. Juggling was the fun focus of one session, and Discovery Hours on memory systems, photography and meditation are in the works.


For my Discovery Hour session, I shared many of my collected activities on codebreaking. With Oscar season just passed and some simmering interest in the Imitation Game, it was a perfect time to talk about the role of codebreaking through history. Even better, my principal and district curriculum director (and my former boss) were on hand to join in the fun:

I was ambitious, trying to fit 4 codebreaking challenges into the hour. In the end, we had just enough time to keep things moving and hold some fun discussions in these 4 areas. Scroll below to download the handouts.

CRYPTOGRAMS – We started with a basic letter-to-letter cipher. I used a long quote from Bill Gates, which almost turned out to be too long – as I felt a time crunch hitting early. But longer quotes allow more entry points, and I couldn’t pull my principla away from the challenge!

CAESAR SHIFTS – Here we used an online applet to explore shifts, and this provided an entry point for modular arithmetic, which few of the students had encountered before.

HILL CIPHER – By now we had established that the first two coding procedures did not seem too secure. I have shared Hill Cipher with students in my classes before during matrix units, and again a cryptography website was helpful in providing some easy codebreaking trials. When I have done these in class, I often develop problems which get around the modular arithmetic issue (it takes longer to discuss than I often have time for) but we were able to squeeze in a 5-minute mod primer.  See below for other Hill Cipher problems I have used.

THE ENIGMA – The cherry on the sundae, and where many students were stunned by the complexity. This online Enigma simulator is one of my favorites – I love the visual of the wiring. So many good questions concerning inverses, how codebooks were traded and how the British broke the code. I left enough time to show Numberphile’s Enigma video, which capped off the hour nicely.

Looking forward to sharing more of what I know in later Discovery Hour sessions, and thrilled so many of my colleagues are buying into the idea.

Inverse Function Partner Share

We’re working through functions in my college-prep pre-calculus class; meaning a more rigorous treatment of domain, range, and composition  ideas than what students experienced in earlier courses. As I was about to start inverses last week, I sought an activity which would provide some discovery, some personalization, and less of me rambling on.

These are the times when searching the MTBoS (math-twitter-blog o’sphere) leads to some exciting leads, and the search for inverse functions ideas didn’t disappoint – leading me to Sam Shah’s blog, and an awesome discussion of inverse functions which I turned into a sharing activity. A great list of blogs and MTBoS folks appears on this Weebly site.

To start, I wrote a function on the board, and asked students to think about the sequence of steps needed to evaluate the function:

The class was easily able to generate, and agree upon a list of steps:

  1. Square the input
  2. Multiply by three
  3. Add 1

From here, I asked the class to divide into teams of 2. Each partnership was then given two functions on printed slips (shown below) to examine: list the steps of the function, and provide 3 ordered pairs which satisfy the function.



Notice that the functions are arranged so that A and B in each set are inverses.  Partners were given two different functions, but never an inverse pair. So a team could get 2A and 4B, but not 3A and 3B.

My plan was to complete this entire activity in one class period, BUT weather took hold. They day we started we had a two-hour delay, and the next two days were lost due to snow, then a weekend. SO, the best-laid intentions of activity, sharing and resolution became activity…..then 5 days later.

As we started the next class day, I asked students to review their given functions (and re-familiarize themselves), then seek out the teams who had the other half of the function pair and share information. So a team which had 2B sought out 2A, and so on.

After the sharing, a classwide discussion of the pairs was then seamless. Students clearly saw the relationships beteen the inverse pairs and the idea of “undoing” steps, and we could now apply formal definitions and procedures with an enhanced understanding. Also, by sharing ordered pairs, students saw the domain-range relationship between functions and their inverses, and this made graphing tasks much easier. I’m definitely doing this again!

Finally, notice that pair 2A / 2B features a quadratic / square root. While we didn’t dive right in at the time, this set the trap for a discussion of one-to-one fucntions and the horizontal line test the next day.