My Students On Opportunity Costs II

This was on the final I gave my students last week:

Suppose a friend offers to take you bowling for your birthday, an option you value at $100. You can also spend $50 for a bus ticket so you can spend your birthday with your significant other, an option you value at $200. You cannot do both options. What is your opportunity cost of going bowling?
a. $100
b. $150
c. $200
d. $350
e. None of the above

A similar question was put before professional economists not too long ago (the scenario was different and there was no "None of the above" option). Astonishingly 78% of those economists got it wrong which is worse than if they guessed randomly (there, 75% would get it wrong).

For my class, I am proud to say, 71% got this question correct. Part of this is surely because they read about the original question in The Economic Naturalist which they read. However, that was at the beginning of the summer semester and this was on the final. Still, it likely had a big impact because I did the same trick last year for my international economic policy class. Only a quarter got it correct then (I repeated the question for the final with some tweaks to make it clearer; 52% still got it wrong). They did not have The Economic Naturalist but they did have the same question twice in a row.

Other theories:

This year I used the Modern Principles textbook (blog here, which I help write). I think having a textbook, especially this one, helped. It gave students another resource to consult and went into more detail than I could in class.

I think this is also the year when I started systematically referring to opportunity cost as the "net benefit of the next best option," not "value of the next best option." Most resources (including Modern Principles) use "value" which I think confuses students into thinking it's about how much I value the option, not how much I gain. In other words, opportunity cost includes the cost of the forgone option, which is why the answer is (b): $200-$50=$150.