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Also, I take it to mean 21% use a client-side A/B testing framework.

You could be performing A/B testing server-side and it would not be detected using this approach (which ultimately boils down to greping the HTML for known strings).

That's how I read it: 21% A/B test, of whom 93% use Optimizely.

I agree, that said, I think it's hard to say the 79% _don't_ A/B test. Could easily be homegrown, or round-robin based, or perhaps it was only included on a specific page.

Yes, especially given the zeitgeist around A/B testing, that seems unlikely.

That's how I read it. Basically 19.5% of all YC use Optimizely.

Yes, the latter.

93% of the YC companies that we found using an A/B tester were using Optimizely. This, however, does not account for server-side testing as mentioned in the thread.