20 Page Level Ranking Factors
And just to clarify, it is not a ranking factor nor mandatory you put the keyword in the meta description. 
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I personally do because it is statistically proven that websites that do, typically rank higher in the search results.

In the matter of Keyword Density, I have seen 0.5% rank #1 as well as over 2.5%. However once you start taking it higher, at about 5% it can negatively impact you for keyword stuffing.

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