h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

But when you go over to the machine to cast your vote you have to enter a four-digit code which tallies with the same number on a piece of paper you just signed, so if *they* want to they could find out who voted for who by matching the two numbers.

It was the same in the UK too - the last time I remember voting there (the 1997 general election I think), my ballot paper was torn out of a book, rather like a book of raffle tickets... and this is where my memory gets a little fuzzy because it's been so long, but I think there was a number on the ballot paper which corresponded to the same number on the stub in the book where it had been torn from, and the number of my voting card was written on the stub, so again, if *they* really wanted to they could match the ballot paper to me.

Harrumph

Yep, your ballot paper is numbered, and you are marked off on the register when it's handed to you, with the number noted. But what can you do?

I refer to the comments in this link just as they are relevant, but take them with as many or as few pinches of salt as you care to: http://www.theguardian.com/notesandqueries/query/0,,-1051,00.html

I don't know whether any of the incidents mentioned actually happened, but I don't doubt they *could*, even though there are reasonable safeguards. The only way to categorically say it could never happen is to make sure your vote is untraceable.