Why are you doing this?
Predictio is an experiment based on the principle of The Wisdom of Crowds by James Surowiecki. The aim is to see whether the central premise of the book is essentially correct, by crowdsourcing a prediction for a future event.
Why are you giving away a prize for a correct prediction?
Because there needs to be an incentive to people both to participate and to attempt to make an accurate prediction. Without some incentive, I don't think we'd get enough participants to make the results statistically significant. And without linking the prize to a correct prediction, there would be no reason to try to be correct - you could make obviously implausible predictions and still be in with as much chance of winning as someone who does a lot of research to try and work out what the most likely outcome will be.
Doesn't that make it a bit too hard to be in with a chance of winning?
Not at all. Even if you did guess completely randomly, your chances of winning would be one in just over ten thousand for the EU referendum prize, which is a heck of a lot better than the national lottery! But, in reality, you're not going to guess randomly, because the most plausible outcomes are clustered around a fairly easy to predict point. Nobody, for example, is going to predict a 0% vote to leave, or 100% turnout - the actual values will be a lot closer to the centre than that.
Anyway, it's free to enter, so the question of probability doesn't really come into it. You might just as well take part if the odds were several million to one.
Why are there adverts on the site?
Because it's free to enter, and someone has to pay for the prize.
What happens if you don't get enough advertising income to cover the cost?
Then I'll have to put my hand into my own pocket, and won't bother doing the experiment again!