For the fifth year in a row, biz school professors Yael Hochberg of Rice University and Susan Cohen of the University of Richmond have ranked the many accelerator programs now up and running in the U.S. as a way of helping making sense of which are worth the time, effort, and, often, equity that founders give them.

It’s no small undertaking. According to their findings, there are currently 160 accelerators in operation, representing year-over-year growth of around 50 percent dating back to 2005, when pioneer Y Combinator first came on the scene.

Some have come and gone. In fact, many outfits don’t survive more than a couple of years. Yet there are often upstarts to replace them, a growing number of which are corporate programs. (Research from Hochberg and Cohen show that 26 such corporate accelerator programs sprung up last year, compared with one or two back in 2011.)

You can see above which programs rank most highly overall. We talked with the professors yesterday to find out how they ranked everyone, and what the difference between platinum, gold, and silver really means.

TC: Explain the methodology you used to collect and sort data about all of these accelerator programs — and why they give it to you.

YH: They give us information about every company and every round about their graduates dating one-, two- and three-years out [from their current cohorts], which is a granularity [of information] that you can’t get elsewhere. And they’re willing to give it to us because we don’t do anything commercial with it. We just do this as a service to the community.

Once they’ve shared the data — which they do confidentially; we sign non-disclosure agreements — we can benchmark them against other accelerators whether or not they [are ranked at or near the top of the heap]. The benefit for them is knowing they’ll at least get back data that’s useful to them.

Full disclosure — this year we supplemented our prior years’ YC data with publicly available data. We didn’t get a spreadsheet from them this year. They have told us that they prefer to be considered a seed fund and not to be called an accelerator. But technically they meet our definition and if we didn’t include them — which was the case last year by their request — it would be . . . odd.

TC: Once you have that information, how do you rank everyone?

SC: We talk with graduates who provide qualitative information about each program, either positive or negative, to understand the relationships that accelerators have with their alumni and to assign them a net promoter score. We also create a total index score to get to a composite number based on five categories: valuation, fundraising, exits, survival, and satisfaction.

TC: You’ve previous ranked accelerators numerically. Now you’ve moved to a system where you’re clustering them into buckets like platinum and gold instead. Why?

YH: Because TechStars and 500 Startups [and their ilk] are great programs and they aren’t statistically different. What we’re finding instead is it’s very hard to distinguish [between them] because their results are similar.

TC: Has anyone either fallen out of the top rankings or made so much progress that it surprised either of you?

SC: Some programs here that have been featured over time have been consistently in the top of the rankings. Plug and Play [of Redwood City, Ca.] is a new entrant; it hadn’t given us data in the past. HealthBox is also new.

TC: Is there a big difference between silver and bronze and gold?

SC: There’s a bigger difference between bronze and silver than there is between gold and silver. Achieving gold is something to be proud of. But these are all top, top, and top programs, not top, middle, and bottom programs.

TC: Do you think there are too many accelerator programs out there?

YH: Yes. We’re at the point where we’re past saturation. A lot of small independent accelerators that have popped up, and we expect that many will disappear in a couple of years. You really have to figure out a good financial model. I think even the most successful [accelerators] are still figuring it out.

SC: The number of what we’d call private or independent accelerators already seems to be slowing. You’re seeing some consolidation in the industry, which isn’t surprising. But we’re seeing more and more corporate accelerators opening, which has been interesting. TechStars run a lot of corporate accelerators, for example. Everyone is taking a different tack.

TC: This seems like a lot of work. Will you ever look to monetize your research?

YH: We’ve never had any kind of monetization in mind. We collect data under blanket non-disclosure agreements and we get [that data] because we keep it confidential. The payoff for us is knowing what to tell our students when they ask us about these programs.

SC: If we were better entrepreneurs, maybe we wouldn’t be academics. [Laughs.]