What is the main criteria for picking the Primary/Focus KW?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Christine Hansen: Google uses Semantic Topic Models, and not Latent Semantic Indexing (LSI) - It is yet another myth. In fact, there is no such thing as an `LSI keyword` patent. And there is not and has never been a magical percentage of keyword usage.
  
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  El Bikito: Christine Hansen Kay this is reassuring. Yeah it didn`t make sense to me if Google wanted article contwnt to sound natural. There`s no way of pinning down what`s natural and what`s not. Which is why I do whatever feels natural.
  
  Again thanks for the reassurance.
  
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  Stockbridge Truslow: Keep in mind that you can pick any word or phrase you want and call it the "Primary Keyword" and whatever tools you have will analyze it based upon that.
  
  Google, on the other hand, will NEVER know what you picked. It just looks at a page and tries to figure out what it`s talking about. The tools are supposed to help you with clues on how to make sure Google picks up your primary thing that the page is talking about, but really - they do a horrible job. They are using metrics and measures that haven`t been useful for 10-15 years in a lot of cases.
  
  Keyword Density is a myth.
  
  Having your keyword first in a heading can help, but it`s by coincidence, not reality. Having your keyword in close proximity to a word that qualifies, clarifies, or gives meaning to the word is what gives is "prominence" - not how far it is from the beginning of the sentence. If you would look at the first to sentences of this paragraph here, the logic they describe suggests that the main keyword is "Having" and that "Having your keyword" is the important thing - but what Google will see from those sentences is that having it "first in the heading" and/or "in close proximity" to something else are the really important parts of those sentences.
  
  The keywords you PICK are just ways to track your success - not a guideline on how to create the content. If you create a page that you`re hoping will rank for the search term "how do you pick a keyword" - then if you pick that, you`ll be able to track how well it does. That`s it.
  
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  El Bikito: Stockbridge Truslow Thank you so much for your insight. I am aware of how KW density and many other 101 practices have been nullified over time with each google update.
  
  However, idk what it is but some of these actually work sometimes. Maybe it`s a total fluke.
  
  Like I don`t do this personally but I`ve seen posts in SEO mastermind group how the admin i think (not entirely sure if it was Scheiler or someone else) tried to exceed or match the KW density of the 1st 3 pages and that apparently helps.
  
  So I wanted to do the same and run a case study which is why I`m asking if KW density is a measure of all the KWs on the page that have SV or like only the PKW.
  
  1h