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As with most of our Crawl Diagnostics recommendations, using the keyword around 4 times is like the Pirate's Code: It's more of a guideline than a rule. That's, in part, why we chose to call it "appropriate" keyword usage rather than "correct." In general my rule of thumb is "use the keyword when it makes sense to use it, without so much that it sounds crazy" which in a standard-size web document of 300 words or more is going to be around 4 times. Including synonyms and other semantically related terms is also good to do, and should be pretty easy if you've chosen a good topic for your keyword. I hope that helps!