Alternatively, a specific stock could work, possibly over a longer time-frame, and I think it would be easier to have an interesting discussion (if that is one of the goals) about it. Someone who doesn't follow the market extremely closely probably won't have anything to say about why they think the market has a certain probability of going up/down next Tuesday, but people should generally be able to form a coherent (even if not particularly accurate) impression of a major company's prospects from general knowledge.
In order to avoid mind-killing, it would be best to avoid AAPL or any company in a politically charged sector (energy, banking, defense, healthcare), but that leaves plenty of room. I would be most inclined to go with GOOG, because everyone is very familiar their products.
Asserting p(GOOG goes up over the next year) > 0.9 doesn't seem that absurd to me. Asserting p(GOOG goes up tomorrow) > 0.9 does seem a bit absurd to me.
I'm not sure whether one is ACTUALLY more absurd than the other, but it intuitive FEELS to me that an economist OUGHT to be able to make long-term predictions about major, stable stocks with 90%+ confidence.
Even if this intuition is wrong (and I'd love to hear if it is!), I think many people will share this intuitive reaction. The goal is after all to select an example which is both factually correct AND intuitively appealing to people. So, an unintuitive but true statement would lack that "gut reaction" value.
When I explain to people how beliefs should be expressed in probabilities, I would like to use an example like "Consider X. Lots of intelligent people believe X, but lots of equally intelligent people believe not-X. It would be ridiculous to say you are 100% sure either way, so even if you have a strong opinion about X, you should express your belief as a probability."
Trouble is, I'm having a hard time thinking of an example to plug into X. For an example to work, it would need the following properties:
Factual question. So no value-laden questions like "Is abortion morally acceptable?" or counterfactual questions like "Would the US have won a war with the Soviet Union in 1960?"
Popular and important question. The average person should be aware it's an issue and care about the answer. So no "My aunt's middle name is Gladys" or "P = NP."
High uncertainty. Reasonable people should be reluctant to give a probability >90% or <10%
No opportunity to gain status by signaling overwhelming support for one side. So cryonics is out, because it's too easy to say "That's stupid, I'm 100% sure cryonics won't work and no intelligent person could believe it." I'm assuming in any debate where you can gain free status by assigning crazy high probabilities to the "responsible" position, people will do just that - so no psi effects, Kennedy assassination, or anything in that cluster. I need a question with no well-accepted "responsible" position.
Minimal mindkilling effect. My previous go-to example has been global warming, but I keep encountering people who say that global warming 100% for sure exists and the only people who could possibly doubt it are oil company shills. Or if I were to try the existence of God, I predict half the population would say it's 100% certain God exists, and the other half would say the opposite.
So what are the important questions that average (or somewhat-above-average) people will likely agree are complicated open questions where both sides have good points? And if there aren't many such questions, what does that say about us?