Teaching Induction and Deduction with Candy Bars

With the Georgia High School Graduation test just around the corner for my 80+ juniors, I decided that now would be a fortuitous time to work on a review of persuasion. Today we focused on logical persuasion, specifically inductive and deductive reasoning.

As I love to have my students collaborate, I divided them into six small groups. Groups 1-3 received snack-sized Snickers bars while groups 4-6 received snack-sized Twix bars. I asked each small group to come up with a Top 10 list to promote their respective bars. They were to make observations about their bars based on size, taste, smell, ingredients, nutritional value, packaging, or even promotional advertising for the bars. Then the groups wrote their Top 10 on the board. (Of course they were allowed to eat their bars).

Then we discussed the differences between deduction and induction. I gave them several examples of each. For example, if I were a CSI working a crime scene and found a pool of blood, spent casings from a missing gun, and a dead body, I can come to the conclusion that a murder has occurred (induction). I reminded them that when they were writing to an audience that was not likely to agree with their position, then induction is a good type of reasoning to use.

Understanding syllogism for a deductive argument was a little more difficult. The idea of a major and minor premise was more difficult for me to explain. We again used some examples: Major premise: Dogs usually have four legs and can bark. Minor premise: Rover has four legs and barks. Therefore, Rover is a dog.

After I assured them that they were already experts at arguing (or at least their parents might say they were), I reminded them that they use this type of reasoning all of the time without even being aware of it. How do they know they will receive detention if they are tardy to class? They know that they must be in class before the bell rings. They know that if the bell rings and they aren't in class, they have to see an administrator for a pass. They also know that they only get one warning for tardies, so any tardy after that results in punishment. Therefore, if a student has already had one tardy, he/she will be assigned detention on the second tardy (inductive reasoning).

Now, back to the candy bars. I asked them to again return to their small groups and come up with one argument about their candy bars using inductive reasoning and one argument using deductive reasoning.

1. Deduction:
Major Premise I: Some people are allergic to peanuts.
Major Premise II: Some Snickers bars have peanuts. (Others have almonds.)
Conclusion: People who are allergic to peanuts should not eat Snickers bars.

2. Induction: (This one made me smile.)
Observation: Snickers bars contain milk and peanuts.
Observation: Milk has calcium, and peanuts have protein.
Conclusion: Snickers bars are nutritional.