https://web.archive.org/web/20130121183432/http://marilynvos...
It contains a clarification that the article omitted from the description:
> So let’s look at it again, remembering that the original answer defines certain conditions, the most significant of which is that the host always opens a losing door on purpose. (There’s no way he can always open a losing door by chance!) Anything else is a different question.