Jan 242012

My last post was about an idea to use old scantrons as a visual aid to build knowledge of the binomial probability formula before the learners actually were introduced to the formula.

Short post: it worked, I think.

Long post: I passed out the scantrons, which immediately brought forth a groan. We just had the final exam last week, and I was already giving them a quiz! Once we got over that part of it, I asked a question.

I was giving them a 1 question quiz. What is the probability they would get it right? All they had is a scantron, no other papers or anything else, so they asked if they were just supposed to guess at the answer. My response was “yes” and quickly they had in hand that the probability was 1/4. It always surprises me how long it takes to get to that point with some learners though.  It is not as quick as I would think. But we all got there within a minute or two, which is faster than normal.

Next, I told them they were taking a 5 question quiz. And I asked the question, “what is the probability you will get a passing grade on the quiz?”

Now the frustration started. They wanted initially to just say (1/4)(1/4)(1/4) = 1/64.  Not so. I killed that pretty quick. But before I did, I wrote it in exponent notation, so they would be comfortable with the idea of exponents having a meaning in the problem. It worked.

The class started guessing lots of things then. (1/4)^3(3/5). That one was creative, accounting for the 3 out of 5 questions. I did not tell them what was right, but we had in the end 5 options the class thought were possible. One of the options was (1/4)^3(3/4). Pretty close to the basic part of the formula, just missing the fact they missed 2 questions, not 1.

So I asked them which of the options written down actually referred to something bubbled on their scantron. After all, 1/4 means something physical related to their scantron. They quickly ID’d the right equation as meaning something consistent,  and very quickly said they needed an exponent on the 3/4.

The last step was asking them how many different ways to get 3 right out of 5. I did the standard counting and after drawing 3 different options someone said, “there must be an easier way.” The class suggested permutations and combinations, and we quickly settled on the combination of 5 C3.

Done. In 15 minutes, we constructed the Binomial Probability formula using nothing but a stack of old scantrons. I then wrote the formula down for them, and they explained what the pieces were for. They made the connection much quicker than before, and I was really pleased with how fast they could plug numbers into the formula.

I remember last year just screaming inside because they could not get the difference between the probability of the problem as a whole, and the p and q values in the formula. They understand that now.

Success, probably. I like it.  Below is a jpg of the notes I made while doing the exercise.

 click to enlarge.

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