Prediction Book is a website for recording predictions. It will email you about whether your prediction comes true when the time for your prediction is up.
It's not a betting site, though it asks you for your percentage estimate of how likely your prediction is to be right. If you make a lot of predictions, this is a way to calibrate how good you are at predicting.
As you may have noticed, there was a recent presidential election in the US. If you have a definite idea about what's going to happen as a result of who won, you can record it and find out how you did in however many years.
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