[DubaiSis]

It's RFM (release figures method I believe), and there's a TON of information on here about it. Google RFM and Greekchat (the internal search function isn't very good) and you should find a full explanation.

But in short, if a girl attends all the parties to which she is invited and lists every chapter to which she was invited to preference on her final ranking card, then, except for unusual circumstances (and not including Indiana) she is guaranteed a bid, even if she only attended one preference party. Nothing is 100% because, as discussed here, there are girls who behave so heinously at preference that the chapter refuses to accept her or something seriously damning surfaces between preference invitations go out and the parties happen. But those scenarios are unlikely at best.

Again, NOT including Indiana University because it is a completely different beast, but there is no set cap on number of girls getting bids, so the number of rushees starting rush has no impact on a girl's likelihood of a bid. Quota just increases. Also, "competitive" is a relative term and really doesn't mean anything where it applies to the likelihood of receiving a bid. Where it does come into play is the likelihood of getting a bid to a particular chapter.