The Security Mentor

For a 100 foot fence, with posts 2 feet apart, how many fence posts do you need?

Most people would say 50. It's actually 51 because of the first fence post in the row. It's easier to see if you think about a 2 foot fence with posts 2 feet apart. Of course you need more than one.

That shows you how easy it is to be off by one when you count things. "Off by one errors" are a really common bug in computer programs, because programmers can make the same mistakes as fence builders.

This also means that if someone wants a computer program to give the wrong answer, they can put in a bug to make it happen that's hard to detect and that looks like an accident.

What can happen if you change the vote count by a single vote in every precinct? Yale University researchers asked that question. Their answer appeared in Commnunications of the Associating for Computing Machinery. Changing a single vote per precinct can swing an election as close as some we've had in real life. In particular, a change of one vote per machine could have changed who became President in 2000.