There are some ways to separate the selection process vs. YC input:
The startups offered YC-funding, who declined it... how did those guys work out, in comparison? (although the offer itself is an important YC input).
Also, the YC guide says they they decline funding to many very good applicants (because YC has limited places and they go to the very very good applicants). It sounds like these guys would have been selected, in some other batch... YC knows who the "almost selected" guys were and could do a comparison.
The startups offered YC-funding, who declined it... how did those guys work out, in comparison? (although the offer itself is an important YC input).
Also, the YC guide says they they decline funding to many very good applicants (because YC has limited places and they go to the very very good applicants). It sounds like these guys would have been selected, in some other batch... YC knows who the "almost selected" guys were and could do a comparison.