I was trying to come up with a framework for estimating the incremental impact of YC. Some of these businesses would have been successful without going to YC. And we can't just compare against the general startup population because YC also selects the best.

So what's a good proxy for estimating the value that an incbuator provides to startups? Perhaps time. Iterating through product ideas, getting introductions, and getting help with legal work & administrivia probably shaves somewhere from months to years off of getting to a good product-often to the point where the product/business would have been abandoned before getting traction.

Let's pick a completely arbitrary amount of time saved-one year. Most of the value of YC's portfolio comes from AirBnB & Dropbox. It's hard to get good revenue numbers, but based on users, nights booked, and other heuristics their growth rates are estimated at around 100%-500%.

If we take an estimate of 350%, that means that we could attribute roughly 70% of the value of YC's portfolio companies to YC itself. I'm not sure if these assumptions are all accurate (especially the 1 year saved figure), but this can give us a starting point to estimating YC's impact on their portfolio companies.

ETA: the amount of time saved is probably the most important variable. If we assume YC saves each company 6 months instead of 12, then YC's impact drops from 70% to 47%. With 3 months, it's about 27%.