CTA based tests are always some of the most interesting, but also provide big gains. One of the most surprising tests was when the button CTA on a form, which was aligned to the CTA on the form itself, lost with statistically significant data to just plain 'submit' and this was after several rounds of prior tests.
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One of the most significant results I've seen was for a client of mine - Switch Video. We changed one word on their call to action button and increased leads generated by 228%. How'd we do it, you ask? Well, first we surveyed their customers to find out what questions they had, and then we made an educated changed based on the survey results. It always helps to start with research so you're not just guessing about what to test.