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In my modeling workshops this summer, we first modeled the money duck, asking ourselves, what would be a fair price for some money buried inside a soap shaped like a duck? We learned how to use the probability distribution model and define its expected value. We developed the question of expected value before answering it.

Then the blogosphere’s intrepid Clayton Edwards extracted an answer from the manufacturers of the duck, which gave us all some resolution. For every lot of 300 ducks, the Virginia Candle Company includes one $50, one $20, one $10, one $5, and the rest are all $1. That’s an expected value of $1.27, netting them a neat $9.72 profit per duck on average.

That’s a pretty favorable distribution:


They’re only able to get away with that distribution because competition in the animal-shaped cash-containing soap marketplace is pretty thin.

So after developing the question and answering the question, we then extended the question. I had every group decide on a) an animal, b) a distribution of cash, c) a price, and put all that on the front wall of the classroom – our marketplace. They submitted all of that information into a Google form also, along with their rationale for their distribution.


Then I told everybody they could buy any three animals they wanted. Or they could buy the same animal three times. (They couldn’t buy their own animals, though.) They wrote their names on each sheet to signal their purchase. Then they added that information to another Google form.

Given enough time, customers could presumably calculate the expected values of every product in the marketplace and make really informed decisions. But I only allowed a few minutes for the purchasing phase. This forced everyone to judge the distribution against price on the level of intuition only.

During the production and marketing phase, people were practicing with a purpose. Groups tweaked their probability distributions and recalculated expected value over and over again. The creativity of some groups blew my hair back. This one sticks out:


Look at the price! Look at the distribution! You’ll walk away a winner over half the time, a fact that their marketing department makes sure you don’t miss. And yet their expected profit is positive. Over time, they’ll bleed you dry. Sneaky Panda!

I took both spreadsheets and carved them up. Here is a graph of the number of customers a store had against how much they marked up their animal.


Look at that downward trend! Even though customers didn’t have enough time to calculate markup exactly, their intuition guided them fairly well. Question here: which point would you most like to be? (Realization here: a store’s profit is the area of the rectangle formed around the diagonal that runs from the origin to the store’s point. Sick.)

So in the mathematical world, because all the businesses had given themselves positive expected profit, the customers could all expect negative profit. The best purchase was no purchase. Javier won by losing the least. He was down only $1.17 all told.

But in the real world, chance plays its hand also. I asked Twitter to help me rig up a simulator (thanks, Ben Hicks) and we found the actual profit. Deborah walked away with $8.52 because she hit an outside chance just right.


Profit Penguin was the winning store for both expected and actual profit.

Their rationale:

Keep the concept simple and make winning $10s and $20s fairly regular to entice buyers. All bills – coins are for babies!

So there.

We’ve talked already about developing the question and answering the question. Daniel Willingham writes that we spend too little time on the former and too much time rushing to the latter. I illustrated those two phases previously. We could reasonably call this post: extending the question.

To extend a question, I find it generally helpful to a) flip a question around, swapping the knowns and unknowns, and b) ask students to create a question. I just hadn’t expected the combination of the two approaches to bear so much fruit.

I’ve probably left a lot of territory unexplored here. If you teach stats, you should double-team this one with the economics teacher and let me know how it goes.

This is a series about “developing the question” in math class.

I posted the following three tweets yesterday, which I need to elaborate:

“Answer-getting” sounds pejorative but it doesn’t have to be. Math is full of interesting answers to get. But what Phil Daro and others have criticized is our fixation on getting answers at the expense of understanding math. Ideally those answers (right or wrong) are means to the ends of understanding math, not the ends themselves.

In the same way, “resource-finding” isn’t necessarily pejorative. Classes need resources and we shouldn’t waste time recreating good ones. But a quick scan of a teacher’s Twitter timeline reveals lots of talk about resources that worked well for students and much less discussion overall about what it means for a resource to “work well.”

My preference here may just mean grad school has finally sunk its teeth into me but I’d rather fail trying to answer the question, “What makes a good resource good?” than succeed cribbing someone else’s good resource without understanding why it’s good.


  • I felt the same way about sessions at Twitter Math Camp.
  • Kurt Lewin: “There is nothing so practical as a good theory.”
  • Without agreeing or disagreeing with these specific bullet points, everyone should have a bulleted list like this.

Featured Comment

Mr K:

This resonates strongly.

I shared a lesson with fellow teachers, and realized I had no good way to communicate what actually made the lesson powerful, and how charging in with the usual assumptions of being the explainer in chief could totally ruin it.

Really worthwhile comments from Grace Chen, Bowen Kerins, and Fawn Nguyen also.

Adrian Pumphrey:

Really, we need to literally go back to questions such as ‘Why am I teaching this?’ ‘Where does this fit into the students learning journey?’ and ‘How am I going to structure the learning so that the student wants to learn this?’ before we even think about where resources fit into our lesson. This takes a lot of time to think about and process. Time and space many teachers just don’t have.

Chris Hill:

Early on I would edit resources and end up reducing cognitive demand in the interest of making things clearer for students. Now I edit resources to remove material and increase cognitive demand. Or even more often, I’m taking bits and pieces because I have a learning goal, learning process goal and study skills goal that I have to meet with one lesson.

Kelly Stidham:

Great lessons in the context of learning around mindset and methods are the instruments we use to “do” our work. But the reflection and coaching conversations where we “learn” about our work are critical as well. Without them, we use scalpels like hammers.

But this work is much harder, much more personal, much more in the moment of the classroom. Can we harness the power of tech to share this work as well as we have to share the tools?

2014 Sep 8. Elissa Miller takes a swing at “what makes a good lesson good?” Whether or not I agree with her list is besides my point. My point is that her list is better than dozens of good resources. With a good list, she’ll find them eventually and she’ll have better odds of dodging lousy ones.

Great Classroom Action

Classrooms are back in session in the United States, which means lots of classroom action, lots of it great.


The blogger at Simplify With Me posts two interesting activities with dice, one involving blank dice, and the other involving space battles:

Once you have your ships, place one die on the engine, one on the shield, and the other two on each weapon. Which die on which part you ask. That’s the magic of this activity. Each person gets to decide for themselves.

Kathryn Belmonte posts five more uses for dice in her math classroom.

Kate Nowak set the tone for her school year with debate about a set of shapes:

Then I said, okay, so here’s a little secret: what we think of as mathematics is just the result of what everyone has agreed on. We could take our definition of “the same” and run with it. In geometry there’s a special word “congruent” where specific things, that everyone agrees to like a secret pact, are okay and not okay. Then, I erased “the same” and replaced it with “congruent,” and made any adjustments to the definition to make it correct. They had heard the word congruent before, and had the perfectly reasonable middle school understanding that congruent means “same size and shape.” I said that that was great in middle school, but in high school geometry we’re going to be more precise and formal in our language.

Hannah Schuchhardt isn’t happy with how her game of Transformation Telephone worked but I thought the premise was great:

I love this activity because it gives kids a way to practice together as a group and self-assess as they go through. Kids are competitive and want their transformations to work out in the end!

Featured Comments

Mary Dooms:

Kate does a great job connecting all the dots by focusing on the learning target at the end of the lesson. It appears all great classroom action positions the learning target there. Now to convince our administrators.

Mathspace is a startup that offers both handwriting recognition and immediate feedback on math exercises. Their handwriting recognition is extremely impressive but their immediate feedback just scares me.

My fear isn’t restricted to Mathspace, of course, which is only one website offering immediate feedback out of many. But Mathspace hosts a demo video on their homepage and I think you should watch it. Then you can come back and tell me my fears are unfounded or tell me how we’re going to fix this.

Here’s the problem in three frames.

First, the student solves the equation and finds x = -48. Mathspace gives the student immediate feedback that her answer is wrong.


The student then changes the sign with Mathspace’s scribble move.


Mathspace then gives the student immediate feedback that her answer is now right.


The student thinks she knows how to solve equations. The teacher’s dashboard says the student knows how to solve equations. But quiz the student just a little bit – as Erlwanger did a student named Benny under similar circumstances forty years ago – and you see just how superficial her knowledge of solving equations really is. She might just be swapping signs because that’s why her answers have been wrong in the past.

Everyone walks away feeling like a winner but everyone is losing and no one knows it. That’s the scary side of immediate feedback.

One possible solution.

When a student pulls a scribble move like that, throw a quick text input that asks, “Why did you change your answer?” The student who is just guessing will say something like, “Because it told me I was right.” Send that text along to the teacher to review. The solution is data that can’t be autograded, data that can’t receive immediate feedback, but better data just the same.

Related Awesome Quote

If you can both listen to children and accept their answers not as things to just be judged right or wrong but as pieces of information which may reveal what the child is thinking you will have taken a giant step towards becoming a master teacher rather than merely a disseminator of information.

JA Easley, Jr. & RE Zwoyer

Featured Comment

Justin Lanier:

I would want to emphasize that the issue is that Mathspace (and tech folks generally) tries to give immediate, “personalized” feedback in a fast, slick, cheap, low/no-labor kind of way. And, not surprising, ends up giving crappy feedback.

Daniel Tu-Hoa, a senior vice president at Mathspace responds:

[T]eachers can see every step a student writes, so they can, as you suggest, then go and ask the student: “why did you change your answer here?” For us, technology isn’t intended to replace the teacher, but to empower teachers by giving them access to better information to inform their teaching.

2014 Sep 4. I’ve illustrated here a false positive – the adaptive system incorrectly thinks the student understands mathematics. Fawn Nguyen illustrates another side of bad feedback: false negatives.

Let’s look at an example of developing the question versus rushing to the answer.

First, a video I made with the help of some workshop friends at Eanes ISD. They provided the video. I provided the tracking dots.

To develop the question you could do several things your textbook likely won’t. You could pause the video before the bicycle fades in and ask your students, “What do you think these points represent? Where are we?”

Once they see the bike you could then ask them to rank the dots from fastest to slowest.

It will likely be uncontroversial that A is the fastest. B and C are a bit of a mystery, though, loudly asking the question, “What do we mean by ‘fast’ anyway?” And D is a wild card.

I’m not looking for students to correctly invent the concepts of angular and linear velocity. They’ll likely need our help! I just need them to spend some time looking at the deep structure in these contrasting cases. That’ll prepare them for whatever explanation of linear versus angular velocity follows. The controversy will generate interest in that explanation.

Compare that to “rushing to the answer”:



How are you supposed to have a productive conversation about angular velocity without a) seeing motion or b) experiencing conflict?

See, we originally came up with these two different definitions of velocity (linear and angular) in order to resolve a conflict. We’ve lost that conflict in these textbook excerpts. They fail to develop the question and instead rush straight to the answer.

BTW. Would you do us all a favor? Show that video to your students and ask them to fill out this survey.

Let’s see what they say.

This is a series about “developing the question” in math class.

Featured Comment:

Bob Lochel, with a great activity that helps students feel the difference between angular and linear velocity:

I keep telling myself that I would love to try this activity with 50 kids on the football field, or even have kids consider the speed needed to make it happen.

Without some physical activity, some sense of the motion and what it is that is actually changing, then the problems become nothing more than plug and chug experiences.

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