Ask HN: How Effective is YC? (For Research Project)

I am writing a research paper answering the question: "of companies that have participated in the Y-Combinator process, what has made some companies jump into a high level of success, while others have just remained at a mediocre success level? Moreover, what defines a high level of success within the startup industry?” I am hoping to interview as many founders as will let me, as well as stir some discussion about this question on HN itself.