For instance, whereas Airbnb can admittedly be seen as a questionable idea (but not outright bad), Dropbox (the idea, before implementation) sounds like a very good idea. Maybe not a $7b idea, but still very good.
Of course, among the YC funded startups, there may be a lot of ideas that sounded bad at first; but if all those startups are in fact financially irrelevant, should they be used to try to build a theory of success?
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It's interesting to learn that if YC funded 10x more startups they would be just as successful (and maybe more, since there could be a big success in the startups that are left out) -- it means that if the selection process rate is around 10%, YC could do without it entirely, with no significant effect to its bottom line.
Also, I took this picture this summer http://i.imgur.com/B30hL.jpg