What should we measure, though? The easiest thing to measure is how many of the startups we fund raise more money after YC. By that test we're doing fairly well. Even before Ron Conway and Yuri Milner ensured the number would be 100% by offering $150k to every startup we funded, we'd got it up to about 94%.
There were 36 startups in the summer 2010 cycle, the last one before Ron and Yuri started funding everyone. Of those, 33 raised more money after YC, 1 didn't bother because they were already so profitable, and 2 were not able to raise money. So 34/36 or 94.4% either raised money or didn't need to. That number is about as high as I'd want it to be. If it were 100%, I'd worry we were being too conservative in who we funded.