The fundraising, the campaigning and the debates are over, and election day has arrived. Two candidates are left on the ballot, and you work as an aide to one of them.
Reports from the polling stations are starting to come in, and you hope to declare a victory soon.
There are N voters, and every one of them votes for one of the two candidates. There are no spoiled ballots. To win, a candidate needs more than half of all N votes. So far M≤N ballots have been counted, and candidate i holds Vi of them, where V1+V2=M. V1 is the count for your candidate.
From past data and careful polling you know that each uncounted ballot goes to your candidate with probability 50%, independently of the others. That is why you think the win can be announced before the count finishes. If the probability of winning is strictly greater than the threshold W, the victory is yours. Just be sure about it, nobody wants a scandal.