This is a great Quora question with a bunch of really cool answers.
A growth hacker's job is to find a scaleable, repeatable way to grow a metric - be it signups, invites, or a certain action (eg - endorsements on LinkedIn). To that end, she looks at the data, finds a baseline and does whatever she can to increase a the desired variable. In order to do this, growth hackers come up with hypotheses and test their hypotheses out.
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