When something doesn't happen very often people say, "Wow! There was a one in a million chance of that!"
What we don't we say is, "There was a 0.0001% chance of that!" Because it is too technical and it is easy to get confused. It's simpler to talk about how often the thing happens ("once") compared to how often it doesn't happen ("a million times").
When we talk about "1-in" we are making that comparison.
Let's say you make 1 sale for every 500 visitors. We could say you have a 0.2% chance of converting a visitor into a sale. But that's a little awkward. So, instead, we say your 1-in is 500 or 1:500 or you would need 500 visitors to produce one sale.
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