If you would tell an average person: Please write down the probability of each event, then they can handle what they want to write.
If you would change prediction book, how would the new system look like? Would it be that someone writes 40-60% instead of 50%? Would they write something different?
We'll need to talk about the probability of the probability and I'm not sure that "an average person" is up for it.
Maybe one can ask "What do you think the probability of event X is?" and then ask "How sure are you of your answer to the previous question? What is the minimum and the maximum reasonable possibilities?"
Given this, you can construct some reasonable distribution over the [0..1] segment, e.g. Beta, and go on from there.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.