For many years I have started the lecture on supply and demand with the following multiple choice question (usually with iClickers at the beginning of the class):
Suppose the government sets the maximum price for a normal doctor's visit at $20, but the current market price is $40. As a result of this government action, doctors will see:
a) fewer patients
b) the same number of patients
c) more patients
In fact, students always answer the question BEFORE attending lecture, as well as provide a short essay justifying their choice, as part of FlipItEcon's Pre-Lecture assignment.
Clearly, if the goal was to arrive at a clear answer right now, many of you may argue that the wording of the question could be more precise. And I'd agree with you, which is why I would never use this question in an exam. But, again, this is not meant as an exam question, but rather as a classroom engagement question. Therefore, the goal of the question is not for them to ARRIVE at the right answer (the goal of the exam), but rather their JOURNEY as they search for such right answer (the goal of the classroom).
Hence, in using this question, I have two goals:
1) For most students to disagree in an answer, therefore having an incentive to talk to each other
2) For the answer to be surprising; hence, to both challenge, and reveal their misconceptions (to me and to them) at the same time.
And the question as written meets those two goals wonderfully. For instance, here is how students in my class this semester answered the multiple choice question:
Notice how this distribution is clear evidence the question meets goal #1 above perfectly, since most students are equally split among two choices. I could even improve that slip during class, by simply telling them not to vote for choice (b) (maybe by saying "b is wrong"), before they vote with their iClickers. By the way, at this point, I have yet to provide students with the right answer, so they really want to know the answer.
But, alas, the point of this post is to talk about goal #2: student's misconceptions about supply and demand shifts. And here is where students' short justification essays as part of their pre-lecture assignment becomes critical. For instance, consider a sample of two of those justifications (again from my student's this semester), arguing (c) as the correct choice: (I have slightly reformatted their answers for student privacy issues):
"I believe choice (c) is correct because:..."
More Patients: Student Justification (1)
"doctors will want to make up that $20 deficit by trying to see more patients"
More Patients: Student Justification (2)
"since the price has lowered, more people will be able to afford it"
As you can see, these are two completely different misconceptions about the effect of a price ceiling: justification (1) is a supply curve misconception; and justification (2) is a shift in the demand curve one. By the way, at this point students don't realize how their logical errors relate to the supply and demand model since they have yet to be introduced to it (which is the main reason this is such a great clicker question to introduce such presentation to them).
But, here is the poll I would like to do with you:
Which of these two justifications (as a percentage of those students choosing (c) as the right answer), do you think is consistently more prevalent among my students: 1 or 2?
Feel free to provide your answer in the comments section below. Here is a clue: the difference is quite large. In other words, usually about 70-80% of my students justify their answer with one of these two misconceptions.