A well-built sales funnel is never complete until every part of it has been tested and optimized. For maximum success, marketers should dig deep and experiment with every customer interaction point. What follows is a brief guide that outlines what things are good to regularly test and optimize—including PPC, media buys, landing pages, and email campaigns. You don’t have to test everything all at once. Start with the marketing activity the produces the highest return and then work your way down.
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Source: Online Testing Essentials: An infographic on what online marketing activities to test.